Playermodel rigging help

I’m trying to rig a model to the ValveBiped skeleton in Blender, but they both don’t fit with each other. Does anyone know how I could make the skeleton fit inside the mesh? I’ve had this problem before with other models but I’ve ignored them and just moved on with other projects I’ve had in mind.

Also, I’m very sorry if I sound stupid right now. I haven’t done anything related to modelling for a couple of months and I’ve already forgotten some of the stuff I’ve learned last summer.

simply reposition the skeleton i dont think its will affect something. i would reposition the skeleton and rig it from there
the skeleton dont have to be on the same position, you can even put the skeletons legs up in the air and twist the head 180 degrees and rig it from there, just do what comfortable for you
animations… they are the thing that affect the skeleton positions and not the model idle rig