Vid's guides on Pinside. Search for topics started by vid1900
Pinwiki- the whole site and the post purchase checklist at http://www.pinwiki.com/wiki/index.php?title=Post-Purchase_Checklist
terryb Soldering guide here on Pinside
Pinball Ninja site http://www.pinrepair.com/begin/index.htm

I agree with all the suggestions above poster made, especially pinball ninja. It's like $20 to get lifetime access to his site. Many of his posts include videos to supplement the write-ups. He hasn't updated in a long time, but the archives have nearly 900 repair logs for games of all generations.

In addition to the above, it's not too difficult -

Start at the lowest level, and work your way up.

For exmaple - A switch stopped working? Start by bypassing the mechanical switch, and short the two wires together. That causes nothing? Check other switches in the row/column. If they all work, then try shoring the proper pins for the switch at the MPU. If that works, then you went too far, and the issue is back on the playfield. If it still dosen't register there, then issue on the MPU/CPU.

Take it slow, go through each step - take notes in case you need to refer back to them later.