There are no more steps.
Enable port forwarding for Port 80 on your router.
Determine the IP address of the machine XAMPP is on
Make that the IP address that port 80 traffic will be forwarded to
Apply settings -
Now the whole world can see what you have placed in htdocs by entering the external IP address of your router in their browser.
I have told you everything that needs to be done. To ensure I had not missed anything, I had tested the instructions 'quickie sure' with a fresh XAMPP install on a clean machine, using a router that was not set for use with a server. Answer - Yes, these are the only steps needed in most cases. Total time - including unzipping XAMPP - about 15 minutes.
Most routers will allow this kind of setup. YOU will have to read up on your router a bit, and understand the very absolute basics of networking.
There are dozens of additional things that you might need to know in order to accomplish this - How IP addresses are assigned?, what is my external IP?, what is port forwarding? do you want to have a regsitered domain? None of them have anything to do with XAMPP, they all have to do with the state of your
knowlege of your
network and what exactly you want to accomplish.
This part no one can do for you - all routers have slightly different settings and menus, we don't know if you have a fixed or dynamic IP address, if your ISP allows port 80 traffic, or what IP addressing scheme your home network has. If none of this makes sense, you need to get cracking on the reading.
The good news is - I told you exactly what to do. In probably 80-90% of cases, it will work just the way you want.