1. I forget what the thing is called, but it's a needle that goes on the surface of the base. You know your base is flat when the pin on the guage doesn't move.
2. I taped the Sandpaper to a glass screen, put my first three fingers ontop of the baseand swirved around in the following motions:
"Front to back"
"Side to side"
It took me about three hours to do.
3. I don't have any links to tutorials, I just went with what I thought was best and it worked out just fine.
I'm sure you can find tutorials in the FAQ section.
Now, if you where doing a BT, 3hrs would be OK, because they use a combo of belt-sanders and wire-cleaners and finish with a butter knife.
But, the base dose look great, and good job!