Help with thessasthai...?

Could someone possibly provide a novice some help with the derivation
of thessasthai from *gwhedh- ? The following is as far as I could get
and I don't even know if know if that bit is right. Any help would be
much appreciated.

1. gwhedh-s- > thedh-s- (gwh before e > th )
2. thedh- s- > theth-s- (loss of voiced aspirates)??
3. theth-s > ??