I've done some half-assed builds. Currently using a diy channel switcher.
Been procrastinating a build of a rangemaster clone (the thing tony iommi and kk downing/tipton used to boost amps on all the 70's records). Ultimately haven't built anything more complicated cause I haven't had the need. Silicon fuzzes and booster pedals are notoriously easy. There are several <10 parts count schematics out there.
Kits are the way to go for a total beginner if you don't know how/where to buy electronic parts and how to look for the right types and such (tayda is the best site for individual parts), and many freely available schematics only give you general ideas of what parts could work, best to buy a kit with tried-and-true values for all the parts.
The amount of electronics you need to know is relatively small, and soldering technique isn't awfully hard to learn if you have a temperature controlled iron. Much like cooking, you don't need to know all the science underneath and still be good at it, but it really helps if you have an understanding of kirchoff's laws and how a transistor works, because most of the famous beginner boost/fuzz circuits are something comparable to exercises in an analog electronics class in my experience. Basically it's only marginally harder than changing the pickups/wiring on a guitar.
One word of advice is if you get kits, get PCB kits. Breadboard/veroboard is harder to learn on. If you do go with vero use lots of space and don't worry about getting the circuit to fit in a small enclosure, bunching parts together as a beginner is a quick way to get stuff shorting out together if your soldering technique isn't great. Always use a hot iron (above 400 F) and apply heat for the shortest amount of time possible to avoid damaging parts when soldering.
There are a number of proco rat kits available, you should try one of those.