Going to tackle the rack and pinion next. Just got a ACDelco rack and im going to change it out soon. Also got those 4 copper washers from Nissan for the banjo connections

Any tips on the replacement of this? I know you should flush the system before installing the new one and then you have to bleed the steering system once the new one is installed. And also tie down the steering wheel so it doesnt turn once rack is off.

How should I do the fluid flush before disassembly and then the bleeding once the new one is on?

There's quite a bit going on by replacing the steering rack. The Haynes repair manual has a good write on this, with cross references to other sections in the manual for all the other items that need to be removed first. Be sure to install new rack mount bushings. Now would be a could time to consider the condition of the rack bellows you currently have as well since you have to dismantle the tie-rods anyways. Obviously, a proper, professional alignment is required after everything is put back together. As for "flushing"; It's a closed loop system and once you remove the high pressure line and the return line to the reservoir, all the fluid in the system will drain out, except for what is in the rack which is being replaced. Bleeding the air out of the system, once the new one is on, is the easy part. Haynes has a good write up on that as well but fails to mention that the reservoir fill cap should be left off while you are bleeding the air out of the system.