do you have a pattern?
if you don't have one, you could rip apart a tight fit boxershort. the important thing is that it has a front and a back seam. it is easiest if you don't have side seams, but if your boxershort does have them you can get rid of them by overlapping (but not too much! picture 1
take the shape of the boxershort over on paper, and just add length. measure the width of your legs every 6 or 7 inches, taking somewhat 10% off the width (you use stretchy fabric!) before you draw it on your pattern.
now the sewing:
I would not advise you to hem the lower eh.. thingies. you use knit fabric, that won't fray (right word?) and if you do try to hem them it wil probably get wobbly.
so, the first thing you do is making the 'leg seams' (using zig-zag or another strech stich if you have one you find more suitable
). you now have two tunnels; the right and the left leg. (another picture; 1
turn one of the tunnels inside out and put them into eachother, good sides on eachother (you want the seam to be on the inside, not
?). now sew the legs together. (picture 2)
the only thing that needs to be done now is putting elastic in the upper hem. for this, we return to the first picture
, section 2 and 3.
use wide elastic! one inch should do, two is most probably too wide.
first zigzag it on the upper side (2) (attaching the elastic to the fabric). then turn it inside (that's why I made pictures, 3) and zigzag it again on the same place (making it look nice
). I made the elastic orange on the pictures.
sorry for the messy pictures, when I was halfway explaining it I realised there should be more
I hope it's understandable.