Piece of scrap dowel, nothing fancy. I just make sure i do a combination of pouring the glue on the dowel itself and on the sides of the mortice, not just in a big puddle at the bottom, that way no fancy applicators needed.
I simply use a cheap artist brush, a small one off course.
Apply glue to the domino as well as a small bit inside the hole.
Not manny residue to clean off, I even rins the brush with water for re-use.
The cost nest to nothing and easy to use.
I like to use older brushes because new ones loose hair, cheap brushes....
Speaking strictly about a DF 500, I use acid flux brushes, they're cheap and disposable. I just make sure to buy the ones with shorter bristles. The longer bristle version often splay open more and then put glue where you don't want it.
I apply a bead of glue to the top of each mortise wall and let it drip down towards the bottom while I apply an even layer of glue to the Domino tenons. Then I insert them into the mortises to spread the glue further.
I put quite a lot of glue into the mortices when applying glue to the first side & everything that squeezes out when I insert the Domino I brush up onto the Domino. The other part gets a bit less glue in the holes and a regular dose on the side.
For my 500 (which is the only domino I have) I just squeeze glue dow then mortise put glue on the edges and spread it either with my finger or a acid brush