Hello guys! Ever wants to install mods? Well you've come to the right place. Here's a clear and full tutorial!
I have been on both system before, so I'm really clear about this!!

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          Mac
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1. Go on Forge (Click)
2. Make sure you're on your right Minecraft version. (1.8, 1.7 ect.)
3. Click on download recommended version ---->Installer
4. Download file.
5. Start a new folder on somewhere you can easily access. (This is not a-must but I recommend you do this because you are gonna get a lot of stuff.) 
6. Put your Forge .jar file there and open it.
7. Choose client.
8. You should find another thingy pop up, just put it into your Minecraft folder too.
9. Open Minecraft launcher.
10. Use the new profile called "Forge". (You have to use this profile everytime you wants mods in your game.)
11. Launch game. (It should take longer)
12. You should see something at the bottom left coner and a new button "Mods" next to Relams.
13. Close game.
14. Find any mod you would like from the internet. (Make sure it's on the version of forge you installed.).
15. Click "Go" from your top left coner on Finder and hold options.
16. Click Library -----> Application Support.
17.Find a folder named "minecraft". (You should be visting this folder very often whenever you want a mod or stuff).
18.Put your mod (It should be a .jar file) in the folder named "mods".<br>
19. Close all the folders.
20. Launch Minecraft (Profile Forge).
21. Click the new "Mods" button on your Minecraft menu.
22. If you see your mod on the list, your done!
23. ENJOY!!!!!!!!

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       Windows
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1. Go on Forge (Click)
2. Make sure you're on your right Minecraft version. (1.8, 1.7 ect.)
3. Click on download recommended version ---->Installer-Win
4. Download file.
5. Start a new folder on somewhere you can easily access. (This is not a-must but I recommend you do this because you are gonna get a lot of stuff.) 
6. Put your Forge .jar file there and open it.
7. Choose client.
8. You should find another thingy pop up, just put it into your Minecraft folder too.
9. Open Minecraft launcher.
10. Use the new profile called "Forge". (You have to use this profile everytime you wants mods in your game.)
11. Launch game. (It should take longer)
12. You should see something at the bottom left coner and a new button "Mods" next to Relams.
13. Close game.
14. Find any mod you would like from the internet. (Make sure it's on the version of forge you installed.).
15. Type %appdata% in your start options.
16. Find a folder named ".minecraft" It should be the first one.  (You should be visting this folder very often whenever you want a mod or stuff).
17. Put your mod (It should be a .jar file) in the folder named "mods".
18. Close all the folders.
19. Launch Minecraft (Profile Forge).
20. Click the new "Mods" button on your Minecraft menu.
21. If you see your mod on the list, your done!
22. ENJOY!!!!!!!!

I hope you succeed on this, if you have any question, just comment down below, and please do have a good day!!
-EstellaEverlot