1. Installing Minecraft
Before installing the mod itself, you need to decide on the version of the game. To play with mods, try not to take the newest version of minecraft, quite often there are no mods for it or they are in development and not ready for play, which is why minecraft with such mods can often crash.
After you have chosen a version, you need to download it in your launcher and run it at least once.
2. Installing the bootloader (Minecraft Forge)
By default, minecraft does not support modifications, so there are various loaders that are installed on top of minecraft and allow you to start the game together with the mods. There are many loaders (forge, fabric, rift, liteloader), but the most popular at the moment is Forge, which we will install.
Most unofficial launchers (for example, TL Launcher) have a ready-made Forge profile, perhaps you are using just such a launcher, to make sure of this, look for a profile with the name “Forge <game version>”, if there is one, then you can skip the installation and go to the next section.
And so, there was no forge profile in your launcher, to install it you need to install java, and then go to this page and download the forge installation file for the version of the game you have chosen. The process of installing forge itself is quite simple, you need to run the installation file using java, click “Ok” (install) and wait a little, after which the installer will notify you of a successful installation, click “Ok” again and the installation is complete.
3. Installing the mod
Each mod is created for its own loader, so you won’t be able to run on forge a mod created for fabric and vice versa (there are exceptions, but quite rarely). After choosing a mod, carefully read the installation instructions on its page, check that the mod was created for your loader and the version of the game you need.
Almost all mods are installed in the .minecraft / mods folder, you can read the instructions on how to find the folder for different operating systems here. This folder is created when you first start minecraft forge, if you do not have such a folder, you can simply create it.
Some mods consist of several files and are packed into an archive (this is always indicated in the instructions), for installation you need to unpack all the files and copy them to the .minecraft / mods folder.
Download the selected mod for your version of the game and copy it to the .minecraft / mods folder, then run the “Forge <game version>” profile in the launcher, if you did everything correctly, you will start minecraft, and the forge version will be written in the lower left corner and the number of loaded mods.
And finally, I would like to remind you that the more mods you put into the game, the more resources minecraft consumes and the greater the chance of errors / crashes / conflicts between mods, so try to make a backup copy of your world before installing each new mod.