I aint reading thru this redundant thread so pardon me if I put up something that was already posted.
Tips:
Take your time: Better to spend hours cutting your hair and do a good job than rush and spend weeks growing it back.
Use weak/dull clippers in the beginning: If you make a slight mistake with weak/dull clippers, then the mistake wont be as visible. If you use those professional joints and make a mistake it will be obvious. This is just until you feel comfortable holding/maneuvering the clippers, then you can graduate to the sharp clippers.
Use two mirrors: Hand held mirror and the bathroom mirror. Obvious reasons.
Use a clamping motion: personally I like to use a clamping motion when I do my outline (versus a tracing motion) cause it gives a sharper looking cut. By clamping motion I mean the clippers are elevated above your hairline then you 'clamp' down by touching your hairline with the clippers. (instead of tracing across your forehead with the clippers.)
You'll get better with time and experience.