This outlines just one way to mod minecraft as there are other ways as I am aware, this way worked for me and importantly applied the mod to all current game saves.
To install a mod for minecraft you need to have 3 things, minecraft installed on your machine, forge installed on your machine, and the downloaded mod file AND importantly all of these things have to be for the correct version of minecraft.
Minecraft game download here: https://minecraft.net/
Forge Download here: http://files.minecraftforge.net/ (please exercise caution here as there are many ways to download spyware if you are not careful)
Forge mods here: http://www.curse.com/mc-mods/minecraft/category/forge
So here is how I did it
- Went and found the desired mod first and worked out what version of minecraft (1.7.10, 1.8.1 etc) it was made to work with....and downloaded it.....took a note of the version.
- Installed the latest version of minecraft and then once logged in opened the launcher and clicked edit profile (bottom left corner) and changed the version of minecraft to the appropriate version that matched the mod.....and then started minecraft and then closed it.
- Installed Forge by double clicking the jar file.
- Ran minecraft again this time changing the profile (bottom left) for Forge....then closed minecraft.
- Moved mod jar file into the minecraft mods folder (in roaming %AppData% folder called .minecraft)
- Ran minecraft under forge profile and the once in went to mods and checked that it was active.
This did the trick for me, however I had a few mod files that did not seem to work and a few false starts with the version of forge not being the right one.
One IMPORTANT point I will make is please make a copy of the users .minecraft folder in %AppData% before doing any of this as people are not going to be happy if you delete all of their saved games.
I hope that helps