Just add up the total of all the items you want (plus shipping) and put them all into a single pledge tier. You’ll also be able to add to and modify your reward through the pledge manager, which will open after the campaign closes. Note that only one Early Bird is available per backer.
Start with the listed reward price for your Spellbook Mini, then increase your pledge for every add-on you’re interested in for that particular ‘Book. You can also use the automated pledge assistant links to calculate exactly what your pledge should be for every Spellbook Mini you build, to save you on some of the maths.
Doom Edition Spellbook Minis are any Infinite Black Special Edition Spellbook Mini crafted in Onyx leather with Maroon foil. This particular combo is exclusively available through the Kickstarter and will not be obtainable afterwards.