I did this on 1.8.9 but may also work with 1.12.2
1. install replay mod to your mods folder (either 1.8.9 or 1.12.2)
2. Install Labymod installer from https://www.labymod.net/download
3. Open the Labymod Installer, if doing 1.8.9 select forge, same for 1.12.2
4. DO NOT add any other addons after clicking next after selecting the version you will use
5. After that, if you have installed Minecraft to a different place or different drive you can select to install it to that different folder/drive
6. it might ask if you want to install the recommended version of forge, this doesn't really matter
7. Be careful of what addons you install for labymod, otherwise, it crashes the game upon starting it.
8. Installing an addon one-by-one seems annoying but can save you a bit of time to find a faulty addon. So it goes, like install addon, if it requires to restart, restart. then continue till the game crashes upon starting
9. To get rid of a faulty addon, go to your Minecraft folder, (default location is C:\Users\Username\AppData\Roaming\.minecraft) then find a folder called labymod. once you have found it, click it. If you use 1.8.9 click *addons-1.8* if you use 1.12.2 go to *addons-1.12* then just delete the addon you last installed.