Crafting is unlocked after clearing 3-2 on the main story and can be accessed by clicking on the 製造 icon on the main screen. Currently located between the Shield icon for the 'Circles' function and the Shop icon for all the in-game shops.
Crafting requires 'Keystones' that can currently be obtained from Daily/Weekly missions. If you roll on a gacha banner and fail to reach the 200 rolls required to pity-collect a character, then the number of rolls you did on that gacha will be converted into 'Keystone pieces' as another source for crafting.
To start crafting, click on an empty '+' slot and then add a Keystone. You will need 1 Keystone or 10 Keystone pieces to fill one ring to be able to start crafting. You may add more keystones to add more rings if you wish but only one circle is required to start crafting. Filling the circle a second time requires more resources but may provide upgraded subnodes, highlighted in yellow.
After you are done selecting your keystones, confirm with the yellow button and sub-nodes will appear. Here is a list of possible sub-nodes that will appear
Upgraded sub-nodes guarantee that the material you get is superlative.
Some sub-nodes might have the same icon as above but have more specific text instead of a general description like the above. For example, instead of 色彩 (Cafe furniture), you could get ゼリーズゲーセンシリーズ (Furniture from the Jelly arcade series). The first one is a generic 'Furniture' while the second one specifies that you will get pieces from that specific set. Make sure to read the node or use a OCR translation software if you are not sure what the subnode says.
After selecting a sub-node and confirming the one you want, you will be taken to a different screen. In this one, you are given the option to add more materials or confirm your craft. Adding more materials by pressing the blue button here lets you reroll with a new set of sub-nodes. Consequently, your final craft will be from the Sub-node you select here instead of the first one. If you don't want to add more materials and are happy with your craft then press the yellow button again to confirm and see how long your craft will take to complete. If you want to rush your craft then you are given the ability to immediately complete it by using a Crafting ticket. Simply press the blue button next to the craft and confirm the prompt to use a ticket and complete it. Tickets can be obtained from the shop at 30,000 credits a piece and other places.