You should now have one piece of cloth, that looks like this in the middle:
\ X/
\/ L1 = one legpiece
L1 /\ L2 L2 = other
____/D \____ D = diamond
\ / X = empty space
L1 \/ L2 __ = inseam
/\ (should be either all insides
/ X\ or all outsides)
If you're unclear on the concept, either follow the directions and compare the results to the diagram, or look at some legwear that has a diamond in the crotch. It is very common in sweatpants.
You're done! Hem, trim, embroider, etc. as desired.
Variations:
I don't believe I've seen anything really corresponding to "belt loops" in the medieval period -- the same function tended to be filled by "points" (i.e. ties that attached the leg-wear to the upper body garment or to a belt worn under the body garment. Pictures of underpants (braies) seem to be best interpreted as having something closer to a drawstring casing.
There is _one_ pre-medieval garment I'm aware of that has belt-loops -- the "Thorsbjerg trousers" from a ca. 5th century burial in Denmark. (See e.g. Margarethe Hald's "Clothing and Textiles from Bogs and Burials" for details.) Here, a half-dozen or so loops are sewn to the waistband of a set of footed trousers and appear to be functionally equivalent to the modern item.
Beltloops can be done as follows: