you can craft weapons after visiting Omicron at the Command Bridge. Please mind that you have to proceed in the campaign first in order to have Omicron on the Bridge!
You also need Blueprints which either can be bought from the store or randomly dropped by enemies in missions.
After using these Blueprints you can craft them when interacting with Omicron. You have to have some materials which can be made by salvaging items!

You can increase your Relic items' PR by fusing them with other weapons and this way the item will gain further enchants.

Furthermore you can change the basic enchants - check the Modify tab!