Before you remove it, set the prop at 25 degrees BTDC on #1 (or whatever your timing says). Be sure it is #1 firing stroke, not #1 exhaust stroke by putting a finger over the spark-plug hole and feeling for compression.
mark on the rim of the mag with a sharpie or a piece of masking tape or something where the hold-down clamp parts touch on the rim of the mag. Sometimes you can see the compression marks in the paint and you don't need to mark it.
Then, while the mag is out, DO NOT turn the prop. When you reinstall it, with the special pin that locks the mag in the firing position, you can pop it in matching the marks to the hold-down studs, put the hold-down blocks and nuts on finger tight, and hook everything up, knowing that you will be really close to the correct timing.
BE ABSOLUTELY SURE YOU PULL THAT PIN OUT BEFORE YOU MOVE THE PROP.
Set the final timing with the buzz box. Normal safety procedures apply, i.e. disconnect all the ignition leads or pull the plugs.