It might be simplier to visit a local dive shop, preferably one that 'tech' oriented, so you can first see, in person, BC's or BP/wings with crotch straps in place, which will give you a better idea of the concept and how it's rigged, you will need to study that configuration and then figure out on your own how to customize your BC, because that Tusa BC is a recreational oriented BC and was never originally designed for a crotch strap. One suggestion, some of the custom BC work I've had done was performed by a local shoe repair shop as they have the gear to sew heavy webbing, however, don't expect a shoe repair shop to actually help you really design your mods as they will know nothing about scuba gear itself and will simply follow your instructions about what to make/sew.