Can a transmission flush on a 2001 Hyundai XG300 be a DIY job? If so what step should be follow in doing so?

You can't really do a purge/flush yourself ...but you could replace the filter and the fuild the old way by going under the car and removing the transmission oil pan and the filter inside and replacing. It's not a hard job, but it is messy and that's why most people have it done at local repair shops nowadays.

I would suggest using only genuine Hyundai filter and trans fluid. This means, go to the dealer and buy directly. If this a high mileage car, go to an auto parts store and buy a can of "brake clean" that has not been de-chlorinated. Spray this chemical on the exposed valve body (inside the trans) to remove any auto clutch residue before installing the new filter.

Remember that you can't really change all the fluid because your torque converter (inside the trans) holds several quarts of old fluid. In reality, all you are doing is diluting the old fluid with fresh fluid. Before you change the fluid, evaluate the old stuff. Does it smell burned, or is it a funny color. Is your trans working correctly before you change anything??

You may have to change the fluid more than once to improve the concentration of old/new fluid.

If your going to just do a transmission flush without replacing the filter, the cost is around $75.00 at most speed shop like jiffy lube. The cost for the transmission oil and filter replaced is around $100.00 and worth it's while and time. DIY, your going to have to raise the car to get under it and have a tray large and wide enough to hold up to 4 liters. Undo the the bolts that hold the transmission pan and let the transmission to leak out, once the transmission fluid stopped leaking out. Drop the pan and remove the filter from the transmission valve body. Replace the filter, clean the pan, and clean the transmission and pan contact point before replace the gasket and placing the pan back. bring the car back down and add 4 liters of transmission oil and idle the car running the transmission through the gears. check the fluid level and add if needed, go for a test drive and check the transmission fluid once again and add if needed. Good luck, keep me posted. Be glad to answer any more questions you may have.