To merge a feather you will need more than one. Feathers come in varying percentages:
10%, 20%, 50% and 100%
Fusing two 10% feathers together will create 1 x 20% feather, and so on and so forth. You can merge feathers by heading over to your feather inventory - which can be accessed by pressing an attraction feather spot on your summoner.
Select the tab that you wish to open, and then select your feather.
If you have more than one of a feather, it will show on the corner a 'x' symbol, and a number.
Four menu buttons will pop up, one being 'merge'. Select this button and it will automatically merge those feathers for you to create the next level.