I am only a hobby baker and have learned a lot trough this site ,I hope the following will somehow help you:
1.- Make the number with plenty of time for drying, if you do not have the number cutter you would have to make a number template. To get the correct size in height put it on top of the pan for the top tier, this will give you an idea.
2.- The other day I saw in pinterest in "tutorials" a very good idea, make a print of the figure cut outs of the size that you would like to put on the cake,tape it down,put a wax paper on top of it, ( you might want to tape it also so that it does not move) and using the sugar medium that you like
( fondant only, 50/50 GP/F, or GP) you lay it on top following your print figure. Here is the link. needs time to dry also.
3.- For the stars and balloons, I cut out 2 stars, placed the stick in between and glue it together to make one of it, this way the stick does not show in the back, that way in case you turn the cake around it would look neat. You also need to let them dry a couple of days.
4.- You need to make edible glue, it is much better than just wetting the fondant.
5.-The banner looks kind of heavy, Being a newbie I would place it as is, but letting it rest on top of the first tier.
6.- the stripes will be a little hard to acomplish, maybe someone else might help you there. The stripes need to be even all the way around.