With the stems built and sanded as described in Making Stems,  the next step is to draw two lines along the outside leading edge, ¹/₈" apart, and centered.  When shaping, don't do any cutting or sanding inside these lines.
 

If you have the stems clamped to the form, try removing the clamps and drilling holes straight through the stems inside the ¹/₈" lines, about 8 inches apart, over the entire stem.  Drill them square to the tangent of the curve.  The drill bit size should be slightly less than the diameter of 2" finishing nails.  Now nail the stem to the form, leaving the heads of the nails protruding slightly.  This gets the clamps out of the way, while holding the stem securely on the form.  Just don't forget to remove the nails as you strip up to them if you plan to use a narrow external stem and glue the ends of the strips together.  You can see this later in some of the pictures.  The nail holes can be plugged and filled after the completed hull is righted.

A Little Theory

When a strip is installed and goes from the first form out to the stem, the stem must be shaped flat to receive the strip and provide a good solid gluing surface.  The angle of the flat on the stem will change as you strip further up.  The forward part of the stem will have a sharp angle until you get to the turn toward the keel, where the angle becomes much more shallow.  This is called the rolling bevel.  I have found it impossible to accurately shape the two sides of two stems by eye, using a spoke shave, block plane, belt sander, rasp, or trained beaver.  The human hand, while a marvel of design, cannot be depended upon for the reproducible accuracy so easily obtained with a little guidance.  Besides, the beaver left teeth marks....

It seems reasonable that if a short portion of the lower stem (sheer end) is first accurately shaped and a strip installed at its final position, the angle for the next strip can be determined from the one previously installed.  It remains, then, for us to shape the stem one strip at a time, each time adding another strip to reference the next.  The strips we will add will be scrap pieces, long enough to straddle both the first form and the stem.  The first strip should be the first real strip, usually the sheer strip.  It will be the reference strip for the rest of the stem shaping.  Depending on the type of canoe you are building, the sheer strip can follow the curve of the sheer to the bow, meaning all subsequent strips will follow the same curve, or be attached level and parallel to the water line.  This latter configuration requires fill strips between it and the top of the stem.  Regardless of the configuration you choose, accurately shape the stem from the final location of its top (upright) end to the area of the first strip.

Shaping the Stem

The shaping tools.  The narrow strip sanding stick is the same dimensions as the actual strips but without milled edges, with 60 grit sandpaper glued on both sides.  The tool is long enough to span between the stem and the first form, as seen below.  The wide sanding stick is the same, but wider.  This one is used to establish the initial shaping, where the first strip is fastened.  The narrow one could also be used here.  Both of these sanding sticks are useful elsewhere on the boat, being used similar to a rasp or file.  The steel piece is a large round microplane, extremely handy for initially hogging off  wood prior to final shaping with the sanding strips.  It works better than a flat version.

I assume you have a flat plane on the top of your strong back, with the strong back a little shorter than the boat and the stem hanging off.  Using a straight edge of some sort, place it touching both the stem and the first form, near the strong back.  Using a dark pencil, draw a line on the strong back in the straight edge location, on both sides of the stem.  This will give you an approximate reference angle to keep while you hog off the stem wood with a rasp or microplane.  The position or angle of placement of the microplane is by eye, using the line drawn on the strongback as a guide.  Don't get too close to the ¹/₈" lines on the stem - you will sneak up on them using the wide sanding board.

 When you are close, use the sanding stick with the plain end against the first form and the sandpaper end against the stem.  Sand back and forth, maintaining good contact with both the form and the stem, until you reach the ¹/₈" line.  You should now be able to lay a strip from the first form to the stem, and have it lie perfectly flat on the stem.  The shaped area should be high enough above the position of the strip so that the next strip will almost fit.  If not, it will be impossible to get behind the bead when you sand for the next strip.  Now do the other end of the boat, and fasten an actual strip in place.  Don't forget the glue on the stem.  (I know a guy who actually forgot to glue all of his strips to the stem!)  Depending on your choice of outer stem design, trim the ends of the strip appropriately, and do the other side.  Make doubly sure your strip ends line up when sighted head on.  You are now ready to complete the stem shaping.

Staple a piece of scrap strip in place, fitting against the first strip, and spanning from the first form to the stem.  Using the rasp or microplane, hog off some more stem wood while keeping the tool aligned with the last strip.  Again, don't go too close to the ¹/₈" line.  When you are close enough, switch to the narrow sanding board and position it on top of the last strip, with the plain end against the first form and the sandpaper end against the stem.  Sand to the line, and also make sure the stem is shaped a half inch or so above to space required for the present strip.  Add another piece of scrap strip, and repeat the rough out and sanding.  When you get up near the keel, the surface area for a single strip becomes quite large.  You may have to reverse the sanding strip and use it like a file, but be very careful to follow the previous strip and sand to the line.

As you are adding more and more scrap strips, you will notice how nice and flat they lay against the stem.  There may be a slight amount of twist required, depending on hull shape.  This is normal when you maintain the ¹/₈" flat straight and true.

When you are done, pull the staples and use the scrap pieces for the other side.  You may have to do a little touch up, but be very careful.  You don't want to generate a curve or rounding on the sanded surface.  To do the fix-up, run your fingers over the sanded surface and check for high spots.  Lay the sandpaper end of the narrow sanding board on the high spot, and press firmly with your fingers.  Use your other hand to push and pull the board, and don't try to use that hand to guide it.  A few swipes should remove any high spots.  Don't overdo it - it took a lot to get this far.

Now do the other side, then the other end.

An alternative to using scrap pieces of strips to guide the sanding stick is to use the actual strips, forming the bevel as you add new strips.  You can hog off most of the wood in an area for three or four strips, then refine the bevel for each new strip with the sanding stick.  When the sanding stick requires a twist to stay flat on the first frame while still sanding to the ¹/₈" line, relax the twist, but keep the edge of the stick against the form and concentrate on meeting the ¹/₈" line.  As long as the sanding stick touches the form and slides along the last strip as you sand to the line, the bevel will be accurate.  There may be twist in the actual strips, but this is normal and the degree of twist depends on hull design.

One important point to be aware of while sanding to the line is to actually shape an area a little wider a bit above where the strip will ultimately lie.  If you shape enough for just one strip, you will not be able to shape below the bead and the cove of the next strip won't fit between the stem and the bead of the preceding strip.  Shaping a bit above the area required will provide sufficient clearance.

More pictures:
 

The first strip in place.  For this boat, I placed the strip at the sheer at the center form, and fastened it level all the way to the stem.  This will require fill strips below it.
 
 
 

Scrap strips stapled from the first strip, working up the stem as each section is shaped.
 
 
 
 
 

The thin shaping strip in action.  Keep the plain end against the form, while bearing down against the stem, sanding to the ¹/₈" line.

 
 
 

Close-up of a newly shaped area.
 
 
 
 
 

Head-on view of the finished stem bevel.
 
 
 
 
 

A better view of the rolling bevel, with actual strips being attached.