In contrast to plugins, TAS-Battle requires a “clientside” mod to enable slow motion in Minecraft.
For that reason you need to install this mod yourself.
Put them into .minecraft/mods/. Remove mods from other Minecraft Versions from this folder, otherwise the game might not start!
Start the Minecraft launcher, select the correct profile and start the game.
Installing for MultiMC
MultiMC is an alternative Minecraft-Launcher that allows you to create seperate .minecraft folders/instances.
A great portion on how to set it up is described in the tutorials section,
the difference is, that you don’t have to create a new instance, you can import an already existing instance into MultiMC.
Click on “Add Instance” in the top left of MultiMC.
Select “Import from zip” and paste https://data.mgnet.work/tasbattle/TAS-Battle%20for%20MultiMC.zip in the field.
(You can also download it from the link and load it as a file from your PC)