Ok, well il try and explain to make it easier.
Say, for example you downloaded a model to overwrite Gohan (gohan's file name is gohan.mdl) and the new custom model you have downloaded is named goten.mdl you must change the name to gohan.mdl otherwise it wont recognise it as the gohan model
once that is done :-
1.) Go to the c:/sierra/halflife/esf/models/player - this is where all the player models are kept, as you can see theres a file for each skin.
2.) Find the Gohan Folder
3.) Make a backup copy of the file already in there (just incase you want to use it later on)
4.) Put your goten model (which you renamed to gohan.mdl) in the Gohan folder
5.) It will ask to overwrite, click confirm
Now when you turn on esf, your gohan model will be changed to goten