Rip up and re-install any weak or damaged flooring with a crowbar, hammer and flooring nails if necessary.
Attach the drain to the shower pan by setting the top of the drain into the drain hole. Place the washer and nut on the bottom of the drain and turn it clockwise with a wrench to secure it to the shower pan.
Set the shower pan in the shower area and push it forward so both sides rest against the wall framing. Check for gaps between the framing and shower pan. Fill the gaps with strips of wood by screwing the strips to the framing with wood screws and a power screwdriver.
Level each side of the shower pan with a carpenter's level. When the shower pan falls below the level mark, insert a wood shim under the base to bring it back up to level. Shim the shower pan as necessary until all sides are level. Remove the shower pan and screw the shims to the floor. Replace the shower pan and check for level again. Mount the shower pan to the wall framing with wood screws and a power screwdriver when all sides are level.