Don't think of putting materia on the scroll as melding. You aren't actually melding like you meld materia on crafters. You don't need to have picked up a single crafter in order to do it. There's no overmelding or anything like that - you keep attaching tier I materia of each type (det, crit, ss, etc) until it locks you out of it, then you move on to tier II, then tier III, then tier IV. Think of it this way: You aren't putting 6 det on your weapon when you meld a Det IV, you're putting 1 Det IV on the scroll - basically, the cap is 31 det materia (7-8 per tier), not 31 det.
This is what the scroll looks like:
My tips:
> Gather your alex. Do your expert roulette daily for a free mysterious map (the quest to unlock the daily is in the general area of the relic guy after you get your animus), gather soldiery to buy mysterious maps from the soldiery vendor, do FATEs, Hunt (to buy alex directly with allied seals), whatever you want to do to get Alexandrite. You need 75. If a meld fails, you do not lose the alex. P.S. You should try to get a buddy for the mysterious maps, as they can be hard solo, and you lose the map if you start it then die and don't get back in time.
> Gather your materia. The cap for crit is 44, the cap for det is 31, and the cap for acc is 44. That means there's 11 materia per tier for crit and acc, and 8 (one with 7, I think the first one) per tier for det. Figure out how you want to set it up here (you can customize the stats by clicking "[customize]" next to Gae Bolg Novus) and then buy or spiritbond the materia you need.
** You can also make spiritbonding gear and do FATEs in it to knock two birds out with one stone - spiritbond for materia while working on alex.
> Double check that you have everything you need in your inventory, and then open your scroll and start melding. Make sure you have extra materia because unless RNG seriously loves you, you're going to fail a couple.
This. You can always stick some accuracy on it if you can't stand spiritbonding or spending money anymore, too. On my server, the Det/Crit III & IV materia fluctuate between 300-500k. I don't know how much I spent as I bought them off and on over the course of 3-4 months when I saw them cheap, but I'd estimate at least 2-3mil. If I had bought every single one at average price all at once, like I calculated before I started, I'd have spent 10mil. It's expensive. I used up all my Savage Aim & Might materia (I had only a couple extra per tier and failed often) and then just threw the rest in accuracy. I ended up with 39 crit, 26 det, and 19 acc on my relic.