Do you have a link or a site that give instructions on how to design a
rain garden, I would be interested in having one of my own.

In addition to that link, I moved into this house a few years ago and the
back corner of the lot was under water often enough that St Augustine
wouldn't grow there. Over time, I poured a few bags of neighbor's raked up
tree leaves in for compost-in-place, then planted stuff my parents gave me
or I found tossed in the garbage: a tiny oleander that is now 6' tall, some
of these 6' elephant ears (once they get going, use a shovel to cut all the
roots 12" around the base and new plants will sprout), different ivy
cuttings from potted plants just to see what would grow.

If you have a low spot, toss in water tolerant plants and see what lives!
I'm in Houston so I tend to only toss in tropicals that can freeze back to
the ground. Even a 2' piece of philodendron stalk someone trimmed and threw
away.

You can get started without having to know which flowers to plant for what
seasons and your gardening zone...plant what you have and worry about
upgrading it later. By the end of the summer, you should know where the edge
is well enough if you want to put in a rock border or somesuch.

John