There are many videos if you Google "2000 Mustang gt auto trans replacement" in your regular browser. Another good resource are the repair manuals at most public libraries...mine keeps an updated set on the reference side, not for lend, so I take some money and copy the pages.
If buying from a salvage yard, get a warranty.
Auto transmission changeout is not that difficult...here are some tips learned the hard way:
--drain the fluid from the trans prior to attempting to remove it (or buy 50lbs of oil sorb and plan an extra two hour cleanup in your schedule)
--drain the fluid from the trans prior to removing the driveshaft (if the car is a little nose up you will get fluid out the tail shaft when the front yoke is removed)
--remember that even with the fluid drained, the torque converter still holds fluid...handle with care
--before unbolting the trans from the motor, remove all obstructions such as exhaust, crossmembers, etc
--before unbolting the trans from the motor, unbolt the torque converter from the flywheel/flexplate and slide the torque converter back towards the trans...it comes out with the trans...remove it and keep it level until you can lay it aside face/front down to not spill fluid
-----when going back together with it, make sure the torque converter is engaged and fully pushed into the new trans
--carefully remove all connection points from trans (shifter, elec connects, cooler tubes, etc) from trans before unbolting from motor
--tools to make it easier--rent a transmission jack...a 3ft 3/8 extension makes getting the bell housing bolts easier, but not essential...did I mention oil sorb?...4 jack stands tall enough to manage trans and jack if you are working on the ground