Before you can do any of this, you must have workshop enabled.

To do so:
STEP 1. Go onto your library and select M:PN
STEP 2. Click the cog icon on the right
STEP 3. Select "releasebeta"
REMINDER:
RESTART YOUR GAME AFTER INSTALLING MODS

STEP 1. Go to the mod you want.
STEP 2. Press the + Subscribe button.
STEP 3. Make sure you’re on the "releasebeta" of M:PN
STEP 4. Open your game.
STEP 5. In the bottom left hand corner, there should be a cog icon, click it.
STEP 6. Select the mod in the menu, press the check box and to finalise, press Apply.

Congrats, you now have a mod installed! :D

Clearly you have Project Nexus 101: How to read installed if you could read this.