The machine you want to connect and control should belong to the same LAN, i.e. you should be able to locally connect between them without accessing internet. (Virtualized LAN might represent a possible turnaround to run the following steps over long distance, but I haven't explored that yet and don't know whether it works.)

You'll find the latest release here.
You need to install OS-specific versions on all the machines you want to connect.

In Barrier, the controlling device is referred to as server, the controlled ones as clients.
Based on that, you should open up the Barrier application and select the right mode on each of the machines you want to connect.

1. Keep SSL Fingerprint option disabled, in this tutorial we'll only run a first timer to check whether the whole system is running properly.
2. On the server machine, go into the configure server menu, add a new screen to the grid.
3. Click twice on the newly added screen, and give it the name that you see on the barrier app opened on the client machine. You might repeat this step for each of the client machines you want to control.
4. Go back on the Barrier server and take note of the displayed server IP / IPs.
5. In each of the Barrier clients fill in the server IP field with one of choice from step 4
6. Run/Reload Barrier on both clients and server, if you see '..is running' on all of them you're good to go!

If keyboard and mouse mirroning does not work, on each machine open up the Barrier window and click F2 to show the logs. If you spot an error message related to missing SSL .pem certificates on the server log while the SSL fingerprint option is disabled on all machines, take note of the path where the log says the .pem certificate should locate, usually the path ends with /barrier/SSL .
You can then try to run openssl req -x509 -nodes -days 365 -newkey rsa:4096 -keyout '<path>/Barrier.pem' -out '<path>/Barrier.pem'.
Now you can stop and run again the Barrier applications and said error should have gone.

Barrier will assign a name to each device, which you can see from the main window.
Some users have found that using simple aliases with only letters and numbers fixes their connection issue.
In order to to that, you must open the server configuration on the server machine and click on a client screen, open up the edit window and fill in one of the aliases. Stop and run again all Barrier windows. Before trying this out, open the log from the Barrier menu and check whether another explicit source of error is mentioned.