How to decide on materia

Much like the title says how does one decide which materia to use and which stat to prioritize? I am currently at end game with my RDM. On my phone at work so I can't get to my gear easily and not sure if there is an armory option like WoW has to where people can see my gear.

I have looked up which stats to aim for which is i do believe Crit, Direct Hit and then Determination in that order? But I am not 100% sure on that.

After that I am lost when it comes to setting up materia on my gear based on those stats. I know how to apply materia to gear that isn't my issue it is determining which materia to use, how many of them, etc. The actual math side of it for lack of better words.

For most jobs, it's a case of Crit > Direct Hit > Determination.

Sometimes there'll be a situation where Skill Speed or Spell Speed will be useful (Usually if you have a DPS cooldown that lasts 10 seconds and thus want to fit 5 skills into that timeframe)

As for actually putting it onto gear, it's simple:

Each item has a cap for how much of each stat can be on said item. This includes the base stats on the item as well as stats from materia.

If your priority is Crit > DH > Det then you put Crit materia in unless you will go over the cap for the item. If you would, then you put in a Direct Hit materia instead.

If you'd go over the cap with both Crit and Direct Hit (Typically if a piece of gear has both of those stats on innately) then you'd put in Determination materia instead.

There can be a little more math involved in trying to perfectly min/max with cases where you're overcapping by only 1-2 points or something, but honestly, it's so minor compared to simply ensuring that you get the full amount of stats from each materia.

Beyond this, there's Overmelding (Or Advanced Melding), which is typically for Crafted items which has an interaction with the 2 grades of materia per expansion (In this case, we use grades VII (+20 of a stat) and VIII (+60 of a stat)). VIII materia can be put into all slots on an item and then you can add one more beyond that via Overmelding. After which, you can only put in VII materia up to the maximum amount of materia per item, which is 5.

For example, if you have say a Crafted chest piece with 2 materia slots on, you can put in 3x VIII materia and 2x VII materia. Whereas with say a Crafted belt with only a single materia slot, you can only put in 2x VIII materia and 3x VII materia.

For me it depends on where my stats currently are, since I play blm for example I want SS, crit, DH. But when I look at my stats it depends on what breakpoints I can or cannot reach. If I can reach a breakpoint for SS, I meld SS, if not, its better to push my crit or DH to a breakpoint. So, I just meld what will push a breakpoint with the current gear I am using.

Melding is incredibly simple to break down:

1. chose (or look up) your stat prioritity. You have done that and it's correct. RDM/ SMN both have Crit > DH > Det
2. with that priority in mind, fill all materia slots with VIII materia while avoiding red numbers. You get a warning if you wouldn't get the full value.
3. if a piece can be overmelded, use VIII materia for the first overmeld slot and VII for all remaining slots. If you're really short on money, VI for the first slot is also acceptable and often a lot cheaper. Escpecially for your weakest stat, which would be Det

Although you can try to optimize in order to hit certain stat tiers, it's generally not worth the hassle and money while still gearing up. You will have to frequently switch around materia and you only have a 40% chance to get back VIII materia once melded.
There's really not much to it. Unfortunately.

Quick and dirty method of choosing which Materia to meld:
1) Crit if you can.
2) Det. otherwise
3) Direct Hit if neither of the above are possible.

This will usually not give the absolute best result, depending on class, but it will be close enough and it will work fine for all jobs.

Swap #2 and #3 around and that priority is fine. DH is marginally better than Det, though it all comes down to substat tiers and the end multipliers. I have a website for that :^)