Crafting in The Surge is covered on this page. Only Armor/Gear may be crafted and it can only be done after acquiring the appropriate Schematic for that piece.
In order to craft a specific part you must first acquire the Schematic for it. Only Gear/Armor can be crafted and each enemy you approach may contain a new Schematic if you sever the apporiate limb. Schematics take not only Tech Scrap to complete, but also upgrade materials. Each slot: Arm, Leg, Chest and Head have different upgrade materials that players can aquire by severing those specific body points on enemies. As the game advances players will acquire higher level upgrade materials that they can use to not only upgrade parts, but players can also craft a higher tiered version of any part from scratch.
Players will be able to upgrade all equipment, both Weapons and Armor/Gear to Mk. IV before requiring Nano Core to reach Mk. V. You will need much less Nano Cores than you used upgrade materials to reach Mk. V so do not worry. Nano Cores can only be found inside containers locked with high Overcharge (50) power conduits and above. You will find these through out the game and players would be wise to backtrack when they can and open them. In NG+ players will be able to upgrade their equipment to Mk. X in exactly the same way, using Nano Cores for the Mk. X upgrade and upgrade materials for Mk. VI - Mk. IX.