You will need a large plastic tray with a drain plug in it.
A fine cut handsaw my choice is a butcher's bone saw, but a fine toothed handsaw will work too.
An old plastic sand filter and several fiberglass batts.
A stainless wire frame to stand the battery upside down on over the tray.
A wooden slat bench will work too.
A fine tooth keyhole saw.
Some tubes of Liquid nail adhesive.
Goggles, facemask-dust type is okay. Rubber gloves and apron, Battery acid is hungry stuff on clothes and hides.
A VERY DANGERIOUS Operation !
Carefully drain the battery acid in to the plastic tray, leave standing upside down for several minutes to drain completely. Don't let the plates dry right out, they will crack and break later.
Lay the battery on it's side and cut around the joint where the top joins the casing. Cut it carefully as not to cut through the pole pieces. Once this is done, use the keyhole saw to cut through the cell walls inside. Cut very carefully and neatly. This makes it a lot easier to re-build them later.
Lift off the top complete with the plate cell assemblies and stand them in a container of distilled water, (clean rainwater could be used in a pinch).
In the bottom of each cell you'll find a heap of grunge at the bottom of the cells, and between the plates. Every time the battery is used or charged some bits fall off the plates and build up at the bottom of the cell case. This shorts out the plates and the cell loses its charge because of the grunge. Dispose of the grunge carefully as it is made up of HEAVY METALS.
Wash out the cells, I use the garden hose for that and get them clean, wipe them dry with old rags (CLEAN ONES) .
Now you can re-assemble. Place a good bead of Liquid nail along the cell walls and also around the outer perimeter joint. Carefully re-place the top with the plates intact and the weight will pull it down to make a good watertight joint. (ALWAYS) make sure that the bead on top of the inner walls are sound--can't get to repair them if they're not!
Note: If the battery is to be used in a bank and not moved a perfect seal is not so important.
Clean it all up and let dry. Re-use the acid drained out, topping it up with more and distilled water as necessary. This will give you another 3-5yrs of life from your battery!
Filter the used acid through a sand filter and then through the fiberglass insulation, Replace in battery and charge. Job well done!
Sulfuric acid is as mean as a wounded grizzly