OK, here we go...
Carried out with a T8 device, so your screenshots will differ a little (different launcher, same principles should apply)
Open the Play Store application on your device and search for Tincore Keymapper
Click on the correct search result and select install
Once installed, you can either open it immediately, or if you're not in the Play Store next time, you'll find the app listed as normal amongst the other applications
If Android worries that the app isn't responding in a timely manner, reassure it with a "Wait" button click.
Once Tincore Keymapper has analyzed your system, you'll have an introductory screen and images to check through.
Once the intro screen is closed, it may appear that the app has vanished. It is now residing in your notification area. For most Kitkat or earlier devices, if you can't see the notification drawer move the Air Mouse cursor to the top left and drag down.
Open the application you want to have remapped keys in, then bring the notification screen down again and click the Tincore Keymapper entry to make it visible.