You can run the autorun.exe from the Blue-Shift CD then search your Half-Life folder for the required .pak file.
pak1.pak for HL
pak2.pak for blue shift and opposing force
Sequentialy ordered pak files replace the models in a the pak file before them.
e.g:
lets say you have a file v_9mmar.mdl in the pak0.pak (the low defenition pak)
and a high-def v_9mmar.mdl in the pak1.pak, the file in the pak1.pak replaces the file in the pak0.pak.
I think that there is also a tutorial on this at TWHL.