We tested our skills in our last house as we knew we would be moving from it at some point. It was a stepping stone house.
We had some basic DIY skills but we watched a lot of YouTube videos by DIY Doctor, Ultimate Handyman and Skill Builder. There are lots of pros out there sharing their knowledge. Watch several videos and see which ones you feel you can trust.
We have installed bathrooms, gutted it and fitted new bath, mixer shower, toilet, sink, tiled, fitted luxury vinyl plank flooring, redecorated, wallpapered, fitted curtain poles, blinds, but left plastering, electrics and carpet fitting to the experts.
Tools cost a lot, borrow if you can, a job will take longer than you think, always start in an area that will be covered up and take your time. The biggest one you can get wrong incredibly easily is tiling, it needs a lot of planning and thought. But it sounds like you are far from that sort of thing.
I drew our extension (after doing research locally online and looking at what other people had submitted, attending Home Building and Renovation Show and getting their magazine for a while too) then the architect just re-drew the same design and submitted it for planning. But it was a simple kitchen extension. I worked out all the unit runs, electric sockets especially for things like the dishwasher, fridge freezer (ie the hidden ones).
What are you looking to do?