Step 1: Converting The Movie Format
This is a simple step. Find a free and good movie/video converter on the internet and download it. Open the converter and then convert the movie format into a format that is playable on your Apple Device. Once you have converted your movie or video, you are ready to move onto the next step.
Step 2: Import Video Into iTunes
Now you have to import the video into iTunes. Select the movie that you converted from the folder that you saved it in and then select it. Open iTunes in another window. Now select and drag the movie/video into your iTunes under the movie tab on the left. It might take a few seconds for it to transfer into iTunes. Then open the movies tab on the left of iTunes and make sure your movie is placed there.
Step 3: Transfer Movie into your Apple Device
This is the final step. This is when your movie can finally be played from your device. Connect your iPhone, iPod, or iPad to your computer using a USB cable. Open iTunes and go to your device which will be located on the left of iTunes. The Summary will open. If you want to sync all your items and stuff on iTunes into your device then press the Sync button on the the bottom right. Otherwise, open the movies tab in the summary and select which movie you want to sync to your device. Once you select it, press sync. Wait a few minutes for the sync to complete.
Your done now! That's it. You can now disconnect your Apple Device from your computer. Unlock your iPhone, iPod, or iPad and go to videos. Now you should see your movie located there if you performed all the above steps correctly. Then go and enjoy your movies on retina display!