Easiest way I know is to buy new ones.
Seriously, though. The crotch is a rough place to try to patch/repair. I would go with some "seal all". Available at Canadian Tire and I have used it all my life to repair everything from rubber boots and rain suits to gas tanks and lanterns. It doesn't harden and crack like other glues so it lasts pretty good.
If it is a hole you can patch without adding too much bulk, you can still buy rubber inner tube repair material in sheets.
Step 1: Put a pot of water on to boil. Ensure that the bottom of the pot is straight and smooth and large enough to cover, plus half again, the repair area.
Step 2: Support the repair area with solid backing, books, bricks, etc.
Step 3: Rough up the waders, clean the area with naptha, cut a ROUNDED patch from the material, draw a line around the patch to outline the repair area on the waders.
Step 4: Apply cement to the patch and to the area to receive the patch, not going outside the line by more than 1/8".
Step 5: Once tacky, apply the patch to the repair area and set the pot of hot water on it. Leave the pot on it until the water is cool.
Step 6: Remove the pot, check the patch for adhesion all around. Apply a thin bead of cement around the outside of the patch and the repair area, sealing it completely against abrasion and peeling off.
Step 7: Function test!
This will nearly always work and has NEVER failed me.