You need to be atleast 10 ranks before the optimal level of the peice of gear. I.E. your weapon is 16 so you need to be atleast r6 goldsmith. Equip your goldsmithing weapon. Go into your inventory select the item. Then select repair this item. It will put you into synth mode. I can't remember if you have to choose the mats in the synth window or not. If not just choose the mat and select main hand. You will repair the item and get Goldsmithing SP. Hope that helps.