Didn't want to put this into crafting section. Hope helps some peeps I just do my tries of a guide

First of what is melding?

Melding is possible on certain pieces on gear that have so-called Materia slots shown as a circle below its substats. It allows you to improve the stats of your gear.
Note that on DoW/DoM always the mainstats such as MND/VIT/STR/DEX/INT are capped if the item is HQ. For DoH/DoL usually stats are not capped because individual melds are to be encouraged.

How does melding work?

To meld gear you need a crafter on the level of the gear and complete the quest Forging the Spirit if you are DoW/DoM/DoL. This quest will give you the possibility to Rightclick/Squaremenu any crafter to "Request Meld" that they affix Materia to your gear.
Or if you are crafter you need to complete the Quest Waking the Spirit to affix the materia yourself into Materia Slots.
Furthermore the Quest Melding Materia Muchly will allow to add materia beyond given slots so that up to 5 Materia can get melded.

-> Every meld beyond given slots will give a certain percentage on sucess/fail depending on how many materia already melded and which grade Materia you try to add

-> At a certain number of statspoints any stat will cap which you will see on a red number in the melding menu.

To meld Materia/Request Melds you need Materia and Carbonised Matter in the Grade of the Materia or higher. All gear below i55 will maximum allow you to meld tier 3 Materia and all below i170 maximum tier 4. Low level gear will accept tier 1-3 materia depending on its level but I don't know the ilvl breakpoints here.

How do I get Materia?

The easiest way to get Materia is buying off the Market Board but definitely not the most effective.

The cheapest way to get Matera is to "Spiritbond" and convert into Materia. DoW/DoM gear will convert into DoW/DoM Materia or Elemental Materia. DoL converts into DoL or Elemental, DoH into DoH or Elemental. Exception here is the Aetherite Ring that only converts into Elementals.
The gear can convert on certain percentages into different tier Materia. Approximately gear level 1-10 always tier 1, 10-20 either 1 or 2 etc (check guides and databases for more info on this if you want a certain Materia of a certain tier).

Tier 4 Materia DoM/DoW also can be obtained from Sylph/Kobolds/Sahagin/Amal'jaa beasttribes as of latest patch the rank 3 quests give you to pick one materia out of 5 random options.

Tier 5 Materia DoM/DoW can be obtained with a weekly drop of Mhachi Matter by weekly completition of Void Ark and be traded at the Kobold North East of the Aetherite in Idyllshire.

Also with Steel and Brass Pirate Spoild from the Diadem you can get Materia if you gather or go dinobashing.

What is about the new Gear?

New raidgear i220+ will have Materiaslots 2 on Head/Hand/Chest/Pants/Boots/Weapon all others will have 1 slot. Like for normal melds you need Materia; Carbonised Matter and a crafter but you can only use those "safe slots" and not meld beyond that.
It is not yet known if i230 Lore items that get upgraded to i240 will lose the Materia but most likely will lose it.
Crafted gear will still accept up to 5 materia!

Which change brought 3.2?

Before 3.2 you were allowed to meld mainstats such as STR/VIT etc until cap into items. With patch 3.2 it has been changed that only the save slots accept mainstats.
Point of this change is the tank statweigts: before 3.2 tank damage was scales to STR so STR stacking was needed and melded STR accessories with VIT or VIT accessories with STR until cap were BiS.
Since 3.2 Tank damage scales equally with VIT and STR which would make fully STR cap and VIT cap too OP compared to "normal" raidgear. After the change if you use a VIT ring with a STR 5 Materia in the safe slot it is slightly "worse" than a i240 ring. For more information on this ask theory crafters please
Items that gotten melded before 3.2 with mainstats keep their effect.