Fretting is a game of thousandths. Most builders buy a commercial
template and or a
jig. The templates are used with a
hand saw, or a
table saw. The investment in tools that may never be used again is hard to justify for most folks. Then the learning curve must be considered. Not only does the chosen slotting method have to be learned well enough (repeat the mantra: Practice On Scrap.) to be used on an existing finished piece but the actual process of installing frets and performing a level, crown, and polish. It is a job only for the seriously handy.
You are wise to take it to a pro.