I'm a bit new to this, but it works fairly straightforwardly:
1) Go to the mod hub, select "Online", then find the mod you want. Click "download".
2) Along the top bar where you selected "Online" go to "download" (Yes, this is different than the "download" you clicked in step 1.
3) You should see the list of mods you downloaded here. In the last category that shows the size of the mod, click it and it will close.
4) Now, along that same top bar go to "installed".
5) When you come to this section after downloading there will be an "install" button in the lower right corner of the window. Click this.
6) With the mod(s) installed now, you simply click the circle to the right of the mod's description, and this puts a green check mark beside them.
7) When finished click "back". From this screen you must select "single player". For whatever reason if you go to single player without coming here first your mods won't load. When you are done playing and come back to the game, I've found that you must go to the mod section every time thereafter if you want to play with the mods you've enabled. Go to the modscreen (you don't actually have to re-check them), just go there real quick, hit the "back" button and then go to single player. Depending on the size of the mods the game will take a second or two, and then will offer you the choice of "Set up game" or "load game". Take your pick and you're good to go!