After opening the cube menu, cube splits in two options, Briefing and Crafting. To save space, you can easily merge it back into one, just make sure neither of them is open on a certain portal.
Portals Briefing will briefly describe what is awaiting within each portal. You’ll be able to scroll trough all portals in mod here, though currently there is only one, so the Briefing closes after describing this one for you. Portals Crafting will allow you to craft portal parts. To do so, you have to fuel it with required items.
After you craft 2-5 parts, you need to merge them into a separate quest scroll-like item, which informs you how many parts does the portal consists of.
Collect the required amount of parts to finish the portal. That’s not all however. Here’s where the fun starts. The portal has 3 difficulties, and, obviously, it’s you who gets to choose which to open.
Remove the sphere of the portal first, it serves as “I choose this”. Once you have only the difficulty in crafting inventory, you can swap between difficulties to choose which you want to play.
Obviously, the higher difficulty, the better rewards. Each difficulty drops different Tier of rewards. The difficulty modifiers are listed at the top of this page. Once you have decided, throw the portal sphere back in and craft the portal.