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- Exalted Legendary Member
It's far too complex to detail here, but visit halomods.com (down at the moment) or modhalo.net. They have tutorials detailing everything you need to know, and I suggest you acquaint yourself with the tag format as it is essential to Halo .map building.
In essence, you need a utility to convert the CE map to a PC map, extract your data then compile it into a PC or CE map from there. Keep in mind that this is illegal if:
a) Locked the map, preventing people from freely extracting tags due to map owner/creator copyright; or
b) The map contains copyrighted data from Bungie/another entity with a copyright claim.
The second reason won't really land you in a courtroom as Bungie are lenient with their content, just so long as you don't sell it or make ownership claims. Take a look at the disclaimers for the Halo 2 content ripped into Custom Edition.
There is no drag-and-drop method available, you need to know what you are doing. Get to know Halo Haking Tools and Halo Map Tool as well as you can and follow those tutorials.