Warning: Mods can do bad things with your computer and your data, so verify the origin and the source code before installing anything. Using the launcher is the recommended method because MinecraftOven has not received updates since 2015.
Using the Launcher
Note: This tutorial relies on the original discontinued version of MCPIL. Follow at our own risk.
You will need to download and install the launcher, if you need help, read the documentation. There is a “Mods” tab in the launcher, if you want to install a mod, simply click the “Install” button and then select its file. If you want to delete an already installed mod, select it from the list and then click the “Delete” button. The mods should start every time you join a world, but this system (like any system) fails, so you can click the “Start mods” button if the mods didn’t start at time: