The biggest challenge I had getting back into it after a 10+ year reef keeping break is that it's a little confusing.
While there are some beginner's guides, they don't really go into detail on why you're dosing a particular fert....it's just dose 1/2tsp of this, 3/4 tsp of that, etc.
While some folks are perfectly content with just following a recipe and not understanding the "why" behind it, that's not me.
The biggest mistake I made was following a recipe that overdosed everything. I started off with a solution that was too strong, without knowing that it was too strong I knew something wasn't right, so I went to dry ferts....using the exact same level of dosing
When the tank hit a wall, and I began investigating and asking questions I figured out that the recipe I was using was way too strong, mainly because I took it off the NilocG website and it was a range for 90-125 gallon tanks or something similar. I was getting different results from the Rotala Butterfly calculator and just got confused and irritated so I spent an evening doing research until I settled on a different amount of ferts.
All of that could have been avoided if there was a guide of some sorts that explained what levels you should strive to achieve, what each component does for the plants, why they're important, and what to look for when testing to make sure you're at optimal levels. This information is pretty scattered currently. But if something like that existed, I'd have avoided many issues.