First- I didn't get a picture of this but you want to draw your cake outline on a piece of cardboard and cut it out- I usually use an old diaper box or something I have lying around. Just make sure your outline doesn't have too many sharp corner indents as they don't turn out that great in cake.
Second- Get some HEAVY DUTY tinfoil that will be big enough to cover your cake base. For my size I needed the big size roll.
Third- Tape a layer of tin foil on each side of your cake base leaving at least 1 in around the outline.
Fourth- cut snips along the extra tinfoil making sure not to cut all the way to the cardboard (I'll explain why in a minute!) Also snip at each indent so you can maneuver your pan sides when attaching.
Fifth- Now for the complicated part- I'm sorry there are great pictures for this. Roll out your tinfoil, take your base and (on it's side) roll it 1 full turn until you have enough tinfoil to wrap around to create the sides. Always add at least 5 inches for indents and go bigger as cutting off excess is easier then adding more. Take that super long piece and fold in half, then half again. Fold again leaving the open side about 1 1/2 inches. (See pic) This will be your cake side so make sure it's as tall as you want- folding accordingly. Then snip about 1 in apart all the way down on the open side.
Sixth- Attach your side to your base by folding the side snips on top and with the base snips. (See pic) Try to be careful as the tinfoil will rip off and you'll be left with holes. Overlap the side a few inches and tape together with masking tape. This kind of tape does best in the oven so please don't try anything else.
Seventh- Take your cake and examine all sides taping where small holes may have occured to ensure your cake batter doesn't leak out. Now make your batter, fill and bake according to directions.
After your cake has cooled you can simply pull the sides off and you already have your cake on a cake board!
Here's my finished cake, I had to fill it and I'll tell you that cutting and moving this huge cake was very stressful. I needed help when splitting the cake but we were able to do it without it breaking! I hope your cake works out!