I  was recently directed to a blog titled Valita's Fresh Folds.  She has a  cute balloon card she got from another source.  I thought I'd make a  smaller version to fit in a standard 4 1/4" x 5 1/2" envelope and share  it with you! 
 First,  let me show you what I came up with for the outside of the card.  I cut  strips of leftover paper using the same colors as the balloons on the  inside of the card.  (I love using up scraps!)  The colored strips were  mounted with foam tape to add visual interest.  A simple stamp from the  "Card Chatter--Birthday" stamp set, inked in black, finished the front.   But let's get to the card base and the inside----the really fun part!
Begin with a 5 1/2" x 10 1/2" piece of cardstock.  Score at 4 1/4", 4 3/4", 5 1/4", 5 3/4", and 6 1/4". 
Fold   your cardstock: Mountain, valley, mountain, valley, mountain.  Notice  how the folds stack together in your hand, as if you were starting to  make a paper fan from your childhood.  Lay the paper back out on the  table and add a strip of Tacky Tape to the inside of the last two score  lines.  Remove the pink backing and refold the card base.
Flip  the card open to the inside.  You will see that two folded strips have  formed in the middle.  Add a strip of Tacky Tape to the inside of both  strips like this. 
Remove  the pink backing.  Add your balloons, cutting the acetate at varying  lengths to make some balloons appear higher than other ones.  After  adding your balloons, pinch the two folds together to sandwich the  balloons neatly in between.
Note: 
I  used my Cricut cartridge "Celebrations" to cut out the balloons, using  paper from the local Michael's.  Why not CTMH paper?  I had twenty cards  worth of balloons to create and decided to make good use of paper from  my pre-CTMH days.
1/8"  strips of acetate were sandwiched between two balloon cutouts to create  one clean-looking balloon.  (If you don't have acetate, re-purpose food  packaging to achieve the same look.)
 
Photos & directions shared by Brenda F. another talented CTMH consultant. Thanks for the awesome idea!