Step 1: Goto the http://www.feed-the-beast.com website and click on Download Now and install
Step 2: Once you installed the Twitch app, login and then click on Mods
Step 3: Click the Minecraft logo
Step 4: Click on Browse Modpacks to access the list of modpacks. You can also search the name of the modpack you want to download.
Step 5: Click download, wait for it to install then click play
Step 6: Once the Minecraft launcher popups DO NOT PRESS PLAY, click Launch Options then click on FTB Infinity Evolved.
Step 7: We want to allocate more RAM otherwise the game will crash. Highlight this area and change it to minimum 4GB or even as high as 8GB.
Make sure it is: -Xmx#GB and the # is the RAM size. This example I allocated 6GB
Step 8: Click the Minecraft name, then click Play.
Step 9: The loading part should take less than 5 minutes. Be Patient!
Step 10: When you're creating your world for the 1st time. I recommend changing the World Type to "Biomes O' Plenty". This unlocks a lot more scenic biomes.
Step 11: Once you load up the world, click on the 4th item in your inventory slot. This is the quest guide book, it will direct you on what items/blocks you need to craft and give you rewards!
Step 12: Have fun! Here are some youtube series I recommend watching