Creating this type of potion depends on your Alchemy skill. However 
if you do not have any skill in Alchemy, take the item that 
increases the attribute (willpower, strength, fire protection, etc.) 
and combine it with a potion (mana or health only). For example, if 
you take an item that increases your strength by 10 and combine it 
with a potion that heals 250 points, you will typically get a potion 
that gives you a +1 (or +2) to strength and heals about 150 points. 
If you take two of the item that increases your attribute by 10 plus 
a health/mana potion, you get a better result (about +6 in the 
attribute). Your alchemy skill will increase this, but with other 
 skills that may be more useful (stone skin, or magic), this is 
generally what you will get.