Navigate to http://pixelmonmod.com/downloads.php
Find the Pixelmon Launcher (Only for windows) and hit install.
Go through you PCs installation for the program. Once installed you should see the launcher on your desktop
Once you run the Launcher, sign in with your minecraft account and click "new profile". Choose the latest Pixelmon
and the latest Forge. See the images below.
Select all of the sidemods, and add them all to the list. You can get additional sidemods from the pixel on website.
Any of your own mods, such as optifine or Journeymap can also be added by clicking the "browse for mods" button.
Add more RAM to the launcher so that you do not lag: The settings button is the gear at the top of the launcher.
Then your done, click the play button.