Apply a small amount of softened paste on the wire* and attach the balloon.

Apply a small amount of glue where the end of the balloon will go. Attach the end of the balloon.

You can paint the wire with gel colour. Dissolve the colour with a small amount of water if there’s need.

You'll find examples of how to attach the balloons in these tutorials:
* This is the wire that you will use for the cake topper. Choose a longer wire than you think you will need, you can trim it later. A part of the wire will go into the balloon. The other end will go f.ex. into the cake topper. Keep in mind that it is not food safe to insert wires directly into the cake.