Here's my instructions for assembling a tunic with flat-felled seams. As with most hands-on things, this works much better in person. Please feel free to email me if you have questions or if I'm unclear.
a. Line up gusset with corner of arm and flat-fell completely.
b. Sew gusset to body, only sew half the flat-fell seam.
c. Line up gusset with other corner of arm. Sew first seam of gusset and then continue on and sew the first seam along the bottom of the arm. Finish flat-felling that seam (gusset and arm)
d. Sew last side of gusset to body. Only sew half the flat-fell seam.
e. Finish the flat-fell seam around the gusset and the shoulder.
That's it. Just hem the bottom, the wrists and the neck line and you have it.
This pattern uses gussets and gores. Love love love gussets and gores.
I started with a pattern from a period garment that placed a gore in the middle of the front panel. I hate placing a gore in a slit in the middle of a panel. I always end up with an ugly pucker at the point. So I experimented with leaving it out. Viola, Sylvie's QnD Peri-oid chemise. This has since evolved into Sylvie's Dress, Nate's Tunic, and Fearghus' Tunic. Patterns not drawn to scale.
All evolutions have been made by me.. they seem like logical steps and end up with a better fit for my Medieval family.
-I- sew these together with a machined flat-felled seam. A how-to is posted here: "Quick&Dirty Tunic Construction". I suppose you could use french seams.. or false-french seams.. but flat-felled seams are just so perfect. It just falls together in a way that really appeals to my inner type-A personality.