Alright, here's the basic breakdown for the forge.
You have the menu for forge-able equipment on the left. Everything that you can make requires recipes initially or some special circumstance to unlock, but once they are they'll be added to the list permanently. Their stats are on the bottom-center and above that the visual.
You'll notice on the right side where the crafting is lined out. Atop, you get your existing ingredients, and on the bottom what's actually required to make the item. Usually it takes a Forge crystal (of varying grades), a high amount of droppable rubber/plastic/latex, and sometimes cores which come in 3 different flavors, Ruby, Sapphire and Soul. There's also a set amount of Mana required for the transaction, you can meet this with your MP on ordinary circumstances! That's always replenishing right? But if the MP requirement is higher than the Mana you require at your current level, Mana Seeds and Mana items can add to the total and allow you to forge them regardless. Overall the equipment crafting system is a vital part of increasing your strength and progressing in the game. It's part of 2 systems, the other which is the Equipment Simulations that will be run by the town cat scientist where Armors can be purified, or you run the risk of 'Being taken over by said costume' instead by losing to it in a fight.