What do you mean "make it leakproof"? Anything can leak. Are you talking about the process of changing the fluid? Or the pan gasket? Or the transmission?
The least-messy way to change the ATF is via the cooler lines using the special machine built for the job. Call some local shops to find which have the BG Products machine. While that's happening, add an external filter as Ford recommends in the TSB in this caption:
The most leak-resistant pan gasket is the reusable one, but it only works with the pan built for it, and your trans might have the older pan. I think it works with most of the aftermarket cast pans, like this one:
It's not that difficult. Contrary to "expert" forum advice, don't use any silicone sealer on either side of the gasket, especially if it's cork. As an RTV sealer rep said, RTV sealer is a gasket replacer, not something you slobber all over gaskets. Otherwise when you tighten the pan bolts the surfaces are so slick the compression either causes the gasket to tear at a bolt hole or squish out in spots. Get both surfaces totally oil free, maybe use a little 3M weatherstrip cement to hold the gasket in place on one side, tighten the bolts using common sense and you shouldn't have a leak. And the post above this one is pretty much worthless since like any older Ford auto trans, a major amount of oil is in the torque converter, you have to pull the 1/8 or 1/4" pipe plug and drain the converter separately.