Transmission repair is too complicated to carry out in a garage. In addition to the need to remove and install many small parts, it is necessary to accurately measure various gaps and install shims of various thicknesses.
If there are problems with the transmission, you can save not only by removing and installing it yourself. Repairing your transmission will cost you more than buying a remanufactured one. If you can't get a rebuilt transmission, at least have your transmission repaired by an experienced mechanic.