This is not an easy task and I don’t think it can be covered as a forum answer.
You need to make sure that you know how to add metaboxes at post types and then how to use the media uploader of wordpress, plus plenty of javascript…
If you are not familiar with metaboxes and the uploader it will take you some time to get comfortable with them. In that case you could leave it in the choice of your theme’s users to use a plugin for this functionality. There are several plugins for this, free and premium.
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