I've been involved in a few swim step and swim platform builds. What are you wanting out of the platform? What size? Will you be carrying a dinghy on it? How much weight do you consider being on it, max weight? What materials would you want it built from, wood, fiberglass, synthetic/composite wood, or combination? Do you want it contoured to the transom? Do you want the bracing above or below the swim platform?
There are/were a lot of good videos on youtube about building simple swim platforms. The ones I have helped with were the vertical wood slate type that allowed for draining and were on custom 316 SS mounts. One I did some fiberglass layups on was foam cored and was glassed to the rear of the boat, a Bayliner Express cruiser.
Here are pics of some that are similar to what I helped with.
The one I helped with was something like this one but had a smooth top and nonslip foam pads adhered to it.