I'm a ham fisted amateur and I managed the rad change on my GTA in one day.
Although it is possible to do the change without removing the bumper, it is silly not to. There are guides with pics, but on short.
Step one -bumper.
Jack up car, place on axle stands. remove wheels.
Remove screws holding front of each wheel arch liner to bumper (2 each side)
Pull back wheel arch liner to unscrew torx screws between bumper and car. (3 each side)
Unscrew undertray securing screws along front edge (about 5)
Unscrew and pop off bonnet catch catch trim and two side grilles.
Unscrew 4 torx screws holding bumper to car.
Unclip fog light wires/ headlight washer jet pipes if fitted.
Place bumper in towels or grass somewhere safe.
Step two the rad.
Unclip or unscrew the upper and lower rad hose clips and drain coolant.
Unscrew the fan and shroud from the back of the rad (4screws I think) tie to something on the engine.
Unscrew AC rad and shroud from from of engine (about 4 or 6 screws)
Tie AC rad up to front cross member.
Unscrew top rad mount.
Unbolt lower crossmember and withdraw downwards with rad attached.
Step three - reinstall.
Yank rad off crossmember. Check dimensions of old rad to new rad.
And as Haynes say, replacement is opposite of removal.
While it is out, de-rust and paint the lower crossmember.
Replace any of the spring clips on the pipes with proper jubilee clips.
Put it all back together and fill with coolant.
There is a lot to do, but getting the AC rad, plastic shroud and car all aligned without removing the bumper is just a huuuuge amount of time and frustration.