Replacing a Shower Stall?

I live in a manufactured home (trailer). The shower stall floor in the master bath needs replacing (it's cracked). It is 2 pieces, so I've got the new base, but I don't know where to go from here.

How do I get the drain out? I can see the screws to remove the walls, but don't know how to do the next step (or what the next step is). Please advise. Thank you.

By Lourie from Yuma, AZ