Yeah definitely take the liner out on the rears, you will need to anyways to get a nut or something on the backside to bolt the new flares to anyway.
It's only 6 of those push in plastic pin things, a door panel removal tool makes it very easy to get them out. Pulling the liner out after is the hard part.
3 of the bolts screw into the fender, the other 5 have plastic nuts behind them that you can hold with your fingers while you spin the bolt out. You can tell which ones they are because the washer is smaller on the 3 that bolt into the fender.
the front flares you can actually get behind the fender to get to the nuts without pulling anything off.
kind of a pain but it's easier than drilling or using a chisel and you may be able to reuse some of the hardware depending on what flares you get.