The guard column should be as close to the column material as possible. Choose identical chemistries, but larger particle sizes.
All vendors are different, so try to use the same vendor for the column and the guard.
Columns are NOT replacements for filters. Keep the guard as close to the main column as possible. Guard columns are not substitutes for sample clean up, they only serve to protect your column from mistakes. Replace a guard regularly. It has a limited capacity and will fill up.