Be happy to...
Fully set up you 29g and fill with water and dechlorinate. Take the time to get your new water temp to the same temp as your 5g. Then take you 5g filter media and place it behind your new 29g media. It should fit, those filters always have extra room for additional media. Though its not necessary, I would also transfer you 5g decor as well because it also harbors some benficial bacteria(bb). Then you put your fish in the new tank set up. You can acclimate them however you wish, I personally would dump them in with the 5g water. That would likely lessen the chance of them going into shock from variances in the water conditions like ph or hardness. Then thats pretty much it.
The old filter media still has enough bb growing on it to process any ammonia or nitrite that your three fish will produce, plus I don't think two guppies and a platy will put out enough ammonia to bring 29 gallons to toxic levels as long as you have some bb in the tank. One important note... make sure you dechlorinate your NEW tank BEFORE adding the media from the OLD tank. Otherwise the chlorine and chloramine will kill what ever bacteria you have.
Then you let your 29g alone for a few weeks, aside from regular water changes. Check the ammonia/nitrite/nitrate levels every day if you can especially the first few days. If something goes wrong and ammonia spikes add ammo lock to keep toxic levels low enough to buy you time to do major water changes. Remember however that ammo lock really only masks ammonia and it will wear off eventually, so if that does happen you must, must, must do huge water changes until the tank cycle catches up. That is worst case, and I have never had that happen using this method but its good to know what to so if that situation occures.
You will know when you can remove your old media when your new media looks similar to the old. Full of bacteria, smelly, full of life, you know what I mean. When you get to that point you can remove your old media and decor and re-setup your 5g without effect the cycle of the 5g (as long as you dechlorinate again). However your bb in both tanks requires ammonia to survive. So, DO NOT return your three original fish until you have others to replace them. I hope that makes sense. If one tank goes to long without fish producing ammonia then the colony will die. So after a few weeks you will have two established tanks.
I hope that helps, let me know if you have any questions. Merry Christmas!
