My shop offered to do the flush but I figure I should look into doing it myself. The entire AC system is being replaced except for the evaporator and the line from the dryer to the evaporator.

Should flush it from inside the car, where the expansion valve goes? Or flush through the line from the dryer? Any suggestions on which brand of flush to use, and how much to use? And I just blow it out with shop air when I'm done? Any tips, tricks, cautions or other info would be appreciated.
Thanks!