tools->(something in the firstl lines with hl) -> decompile a modelbla bla bla, something like this.
Select the .mdl file you want to replace.
it decopmiles the model, then load your mesh, and then : file->import-> smd file
import the .smd file of the .mdl file (its mainly the same name as the model name you decompiled.
Example: goku.mdl has his refrence smd as goku.smd
load it, scale,rotate,move your mesh almost in the same size and porporation as the goku and delet the goku objects,mesh, then rig your mesh to its skeleton, save it as the goku.smd and then
tools->hl shizzle ya know->compile a .qc file, select the goku qc (its only 1 qc in 1 *.mdl) and it will compile the model to a .mdl file.
if a error comes it means you have to fix the problem, there are several ones.
As you copmiled it withouth any problems you are able to play/text it ingame.
its actually not a noobish question, copmilings are complicated mostly. but the hl compiling is one of the most easiest in my opinion.
good luck!