Before you get started rebuilding your transmission, you'll need a few parts and tools. You'll need the appropriate transmission rebuild kit for your TH400. Depending on what type of kit you buy, it should come with at least new steels and frictions. Usually included are all the bands, clutches, seals, sealing rings, gaskets and a filter and drain plug. Most higher end rebuild kits will include a new shift kit as well. An automatic transmission shift kit is only needed if you want the gears to shift sooner, but it can also reduce transmission wear by reducing or eliminating shift overlap. The better kits will also include new valves and valve springs for the valve body.
A transmission repair manual is a must. We recommend one for your specific model from ATSG (Automatic Transmission Service Group). Every automatic transmission will need a universal clutch spring compressor as well as snap ring pliers and pic sets for removing valve guides.
Of course you will need to remove the transmission from the motor and chassis. If you're not familiar with this, the first part of the 'Basic Engine Building' video covers this and more. All of the automatic transmission rebuilding video's out there start off with the transmission already out. So be aware if you aren't familiar with removing the transmission. You will also need a good work space and transmission stand. It's best to have it able to rotate around for removing and installing components.
As you disassemble your transmission, make note of any obvious wear to parts that may need replacing. Keep the internal parts in the order they were removed to keep track of everything.