First a confession -- I don't know how to sew. Beyond fixing the occassional button, I'm all thumbs when it comes to needles and thread, much less sewing machines. But we have some very crafty sisters in our community (and men too, following the Sunnah of the Prophet) who do mend and sew their own clothing. For you, here are some resources that may be of help.
Using Common Commercial Patterns:
A number of commercial patterns (such as Simplicity) can be used or adapted to meet Islamic clothing needs. Keywords to look for when searching for patterns include "caftan." You may also find patterns you like when looking through collections which are considered "costume" or "vintage" designs, or even patterns produced by modest Christians.
Islamic Prayer Outfits:
Dresses and Overgarments: