Well, basically:
- Download the vanilla launcher (minecraft.exe) from minecraft.net
- Start it, set up your login information, you will be presented with the main screen and a big "Play" button
- On the bottom left, click "edit profile"
- In the edit dialog, find the dropdown menu for "use version" and select the vanilla version you want
- (You can also enable older alpha/beta versions or newer development snapshots here)
- After you have selected your desired Minecraft version, launch the vanilla client, then shut it down again
- Go to files.minecraftforge.net and download an "installer" for your desired Minecraft version (may not exist for 1.4.x or older)
- Run the installer and simply let it work, you don't need to do change anything
- You can install any number of forge versions in parallel without problems, just run their installers
- Open the launcher again; on the lower left you can now select a second profile named "Forge", you will be asked if you'd like to use your already setup login information
- You can also elect to make a new profile, or use your original profile simply by choosing your desired Forge version instead of vanilla in the "use version" dropdown
- Each profile gives you the option to specify its own install directory anywhere you like, this is highly recommended
- You can also specify JVM arguments if you like
- After setting things to your liking, start the game with the profile you configured; Forge/FML will do its initial setup, and if specified, the new install directory will be created. Exit the client again
- There will now be the required "mods" and "config" folders in your install directory; just drop all mods and premade configs you may have in there. Remember, 1.6+ no longer has coremods, everything goes into "mods"
And that's it. Making a new modpack is as easy as making a new profile and pasting in the mods. Switching between modpacks is as easy as switching between profiles. Upgrading Forge or trying out vanilla dev snapshots is as easy as changing the "use version" dropdown. It looks bland at first glance, but that launcher is super-powerful, easy to use and built with modding in mind.