lets say you download a ssj3 goku model, and its titled ssj3goku. just go to c: sierra/halflife/esf/models/player fine the ssjgoku folder. and copy that to your desktop, you should backup any models you replace, just a tip.
after youre in the ssjgoku folder and the ssjgoku.mdl is gone, paste the ssj3goku.mdl and rename it ssjgoku.mdl
in shorter terms. you have to rename the models to the model that youre replacing. and put it in the folder of the character youre replacing.