How do i build a port selector?

i only have to com ports on my computer, and the problem i
have is that i have 6 devices that use a com port. i know you can
buy these things, but i'd like to have a go at building one myself

You can multiplex you ports using switches or microcontroller but using this technique if you want all your devices to be operated at same time then its not possible. It can operate any two devices at a time. This is going to be a big-messy job.

The easier solution would be to buy a PCI based COM PORT Card having 4 Com Ports. Wiht this you'll have 6-ports for all your devices and they'll work simultaneously.

i only want to use one at a time, any chance of a diagram
i new to electronics...

Here I found something useful. Check it out.
https://www.netadmintools.com/art40.html