I leave the knife sheathed, and wrap everything, except the part to be molded, in a wet cloth to prevent the kydex you don't want to heat gets too hot. Then, preferably, use a heatgun from upclose, and away from the surrounding kydex to heat the part you want to mold.. when the kydex starts to relax, it's time to mold..
When you're doing a thumb-push it's very important not to influence retention..
to influence retention (make it looser very often) I heat the kydex, and when it's relaxed I slowly start to sheath and resheath the knife till I feel the kydex starts to stiffen.. Just before that I push the knife in completely, and, with the last remaining flexibility of the kydex, form the retention..
It takes some practice to get the hang of it, and very often you have to do things over again, but after a while things will go much easier..
To make the final mold in one time it's very well possible (not with all knives) to mask the parts you want wider with small pieces of tape..
You could make the sheath flare open like this too. I have used electrical tape succesfully. Duck tape or gaffer gets gooyie..
