oke a little example
if you want to replace lets say the cell model
you goto to the esf model map ( C: \Program Files\Valve\Steam\SteamApps\Account\half-life\esf\models )
Click on the player map, then open the cell map and replace the model of cell with the model you want to see ingame
Remember the new model has to be the same name as the old model otherwise ESF doesn't know what to use
owh and a big PS.. remember to make back-ups!