1. Generally speaking you can accomplish 1000 gold base price per day if you craft while adventuring. Only one item per day can be made! (You can craft three scrolls or three potions as long as the base price <=1000).
2. If you dedicating an entire day to crafting, get 2000 gold accomplished. (You can craft six scrolls or six potions in a day as long as the base price <=2000).
3. Player spends 50% of the full cost of the item (or when upgrading, 50% of the difference between the base item and final price of the upgraded item)
4. Have the proper feats and proper spells needed. A scroll of the needed spell is fine. (usually just google search the item you want to figure this out)
5. Make the proper crafting or spellcraft check. If making multiple potions or scrolls, make multiple checks. A natural one on the roll does not equal auto fail nor does a natural 20 equal autosuccess.
You can choose to take a 10 on this check, or roll for it.
6. If you roll for it:
If you fail by greater than 10, the error causes you to lose all gold you spent and you can choose to try again.
If you fail by less than 10, you lose half the gold you spent and you can choose to try again.
If you succeed, item is created successfully.
If you succeed by greater than 10, roll a d100. Getting a >90 means the item was created masterfully and the GM will add some type of bonus to the item. Not applicable to consumables.
7. Example. Crafter wants to create a ring or protection +1. Base cost is 2000. Since he is adventuring at the moment and just spends time here and there crafting, it takes 2 days to make. He pays 1000 gold. He has the forge ring feat, and has the ability to cast shield of faith. He makes a spell craft check against DC 6 and gets a 20. Since he beat the DC by 10, he rolls a d100 and gets 95. The GM adds a special quality to the ring as it is a masterful creation!
A note about DC's:
The trickiest thing about the process is figuring out the right DC.
For most items, the DC is 5+the caster level of the item. For some items, you can find the DC by searching online. When in doubt, ask GM for help.
Here are some costs and DCs associated with adding +1's and 2's to armor and weapons
For simple armor and weapons, in terms of price, upgrades only cost the difference.
+1 = CL 3 DC8 weapon cost: 1000 armor cost:550
+2 = CL 6 DC11 weapon cost: 4000 armor cost:2500
+3 = CL 9 DC14 weapon cost: 9000 armor cost:4500
+4= CL12 DC 17 weapon cost: 16000 armor cost:8000
+5= CL15 DC 20 weapon cost: 25000 armor cost:12500
Looking for a specific bonus for weapons and armor?
http://www.d20pfsrd.com/magic-items/magic-weapons/magic-weapon-special-abil ities/