rear bumper has two plastic tabs right inside by rear lights that have to be turned 45 degrees to release rear bumper (i think its the last bits you need to remove) they are a pain to get to if you have big hands so long needle nose pliers of some sort to get them, undertray bolts may snap, shouldnt have a problem fitting back on if they were on ok in the first place,
check out the guides section there is removal and fitting posts for each bumper