Scratchbuilding is not just one process. Its a variety of skills that get applied in a variety of ways depending on what you're trying to build. are you scratch building a specific part, a specific craft? are you building something completely new? Are you making something all greeblied up using kitbashing, or are you going for something smooth? As for a tutorial you can follow the work of others and you'll see the results but rarely the techniques employed.
Most people learn to scratch build based on trying to improve and existing model kit that they are not happy with... for me that model was the old MPC Millenium Falcon. I just hated that the model did not look like the picture on the front of the box.... or even close. I spent a lot of time raiding car and tank kits adding "stuff" to the side walls trying to make it closer. eventually I started cutting the model apart to get the shapes closer. Learning how to reinforce areas with styrene, putting to close the seams, sanding to reshape and smooth areas, priming painting wash rinse repeat. until I eventually had something that I still was not happy with but most everyone else thought was great.
I guess what I'm trying to tell you is there is no real tutorial - you just kinda need to feel your way... and once you get to a spot where your stuck as specific questions about what to do and get 15 answers... one of which will "click" for you and you try and likely get so -so results... but you'll learn a new "trick", and the next time you'll do better. We learn by doing.
Jedi Dade
