As fine and tough as possible.I usually use 1/4 strength of artificial sinew.Small a needle as possible with the stitches at least 1/16" apart.Then after stretching and softening is done I cut out old thread and holes also then replace that thread with real sinew that's wet when sewn in with the same needle so it will tighten that hole really well.
From the hair side you won't see it one bit.
