You can Fuse weapons together as soon as you have two of the same weapons (with the same name).
Once you have selected the weapon you wish to fuse, select the Fuse tab and press the Fuse button
To fuse a firearm up to a higher-grade, you must fill up the Fusion EXP gauge.
You can incrementally fill the Fusion gauge by using firearms with the same name (AWM example below) or by using Fusion Cards.
When the gauge is full, a preview of the the new Star Grade will appear (example seen below, 2 stars to 3 stars). -- NB this fusion not only filled the fusion bar but added a further 512 fusion exp. Tap on the Fuse button.
The Weapon Grade for this AWM has now increased and so has it's Max Level potential from 20 to 30.