It shouldn't leak if you use a quality replacement gasket.  Remove the
drain plugs from the transmission pan and differential and drain the
fluid.  Then reinstall the plugs with new washers.  Next, unbolt and
remove the pan and unbolt and remove the filter.  Clean the old gasket
from the pan and transmission.  Install the filter, new gasket and pan.
Add new transmission fluid through the dipstick tube.  Expect about four
quarts.