Complex sentence: Tywin's vanguard was led by Garlan Tyrell, who was wearing Renly Baratheon's armor, leading on Stannis's side (and loyal to Highgarden) to think Renly himself had risen from the dead.
Split into simpler sentences: Tywin's vanguard was led by Garlan Tyrell, who was wearing Renly Baratheon's armor. This convinced those on Stannis's side (and loyal to Highgarden) to think Renly himself had risen from the dead.