Depends. The mods made for Retail HL2 are installed like regular source mods.
Beta mods have a different proces. If you want Beta mods to even work at all, make sure you have
Leaknet installed [If you downloaded the
Mega Build then you're fine].
Just download the mod you want [I'll use the
Borealis Mod as an example]. The mod will come in a .rar file or something similar, so get your WinRAR/7-Zip ready.
Open the .rar file. There should be a folder inside. Extract the folder into the folder where you installed the beta, this is where the hl2.exe file for the beta is located. After you extract it, open the ModLoader.exe file that comes installed with Leaknet and select the mod you want to play from the drop down menu.
Hope I helped. If you have any more questions feel free to ask.