There was awhile when Stabby Stabby would play on our payload server.
THAT was a nightmare.
I use to play with him a lot! He was my sorta teacher at a point. I have some pictures when we use to chill.
If you're wanting to get better at spy, learning from other people about techniques is very good. There use to be a show during HL season where plat spies would get together and talk about issues revolving around spies and what to do during certain situations which was extremely informative. There was a question from someone from the lower levels of HL which was "How do I get better?".
Note: I'm not wanting to take over thread! Just wanting to past some of my knowledge for those to get better!
The answer is practice and having an open mind to learn. If anyone has ever played LoL or Dota, I consider the spy as the jungler. You are there to assist your teammates and to make very important decisions (Sometimes even split second!). You're an extremely important player if you get the chance to spy because you can do so much and sometimes have too! When I played spy for HL teams or use to be a coach before I "retired".
I always told my students to:
1. Be Vocal! You're the eyes and ears of the team and you can see things that you're team can't such as sentries, enemy positioning, and much more. Call things out when you see them!
2. Trick stabs aren't important.
3. Learn the kill priority order.
4. and more.
A lot of it is mental thinking which I consider it's own separate lesson/book.
For the actual stabbing, I recommend any TR_walkway maps. I use to sit there for hours and practice walk by stabs, air stabs, trick stabs and more. It's an extremely useful map for spies wanting to learn and helped me from the mechanical side a lot! Just remember that this won't only help you but also playing on actual servers or teams to put you in scenarios. Good or bad, you'll learn from them.
These are just some of my tips. I can't tell you all of them cause that's where the fun of learning from other people and creating ideas come from! I really recommend talking/learning from other people. It's a huge bonus for spies in training and can help you down the line by picking up things easier. Hope this helped!