Step one would be to go to wowhead and look up items to see whats best in slot. You'll have to compare 78 and 79 required items and decided if its worth it to get up to 79. Remember that it will take more rating to get 1% crit, haste etc. when you level up to 79.
Step two would be to get to that level and turn off XP.
Step three would be to start acquiring gear. As others have stated you can get around the dungeon finder restrictions but you need friends. People will probably be reluctant at first when they see a 78 or 79 but if you explain that you are a twink and get some gear to do reasonable damage it will be easier. To first start out I would try paying a tank. If you advertise in /2 that you have a tank then people will be more likely to go with the plan than wait in queue. To save time on travels you can set your hearth near whichever of the two dungeons you are running. You are a mage and can portal to your capitol city anyways