There are many reasons one might want to brush-on cyanoacrylate (CA). And it is entirely possible to dispense some glue onto a brush or the workpiece and spread it using a stand-alone brush, as one would with paint. But it is more convenient in many cases to have a brush applicator, so i will show you my simple way of creating one. I'm sure you can take the concept and make it your own through customisation.
The fundamental concept for this instructable is to use heat shrink tubing to clamp the brush's bristles in place, and also clamp this 'casing' or ferrule to a glue dispensor, channeling the glue onto the brush head.
The glue has to be able to flow through the brush head. For this reason, the applicator might only be useful for thin (non-viscous) CA glues. However using thicker bristles in the brush head may aid movement of thicker glues between the bristles.

Before we get to stepping, here's what you will need (also pictured):
- Cyanoacrylate glue with a nozzle tipped cap or lid.
- Hair of some sort for the bristles
- Heat shrink tubing (a.k.a. “heat shrink”; or just “shrink”) used in electrics and electronics
- scissors
- point heat source such as a cigarette lighter
- sewing thread.

I have an instructable on making (paint)brushes. The following steps are a foreshortened version of the instructions from that source, since the process is the same. You may like to view the full brush instructable though - https://www.instructables.com/id/Make-brushes-such-as-for-painting/

Step 1: Prepare the Parts

Select a bunch of hair at least four times as long as you intend the final bristle length to be. The bunch should be about half as much as would fit into the heat shrink tubing you're using, since you're going to double it over. I use my own hair because i have lots of it. But other options such as horse hair will work.

Cut a piece of heat shrink approximately 40 mm (1 1/2 or 2 inches) long. The heat shrink that best suited my glue bottle's tip was "2.5 diameter".

Cut a length (about 200 mm) of thread. Ideally this should be some man made fibre, since CA reacts with cotton exothermically. However cotton can be used.

Step 2: Create a Brush Head

Tie the length of sewing thread around the bunch of hair at the mid point, and pull the hair into the heat shrink ferrule using the thread. Ideally the fit is snug, but not too compact.

Heat the shrink tube -only where it covers the hair. A small flame such as that of a cigarette lighter works for such local heating. You want to apply as little heat as possible, so pass the heat shrink covered region through the tip of the flame a few times, until you notice it change shape. It's not a problem if the tubing shrinks a bit directly behind the hair.
When the heat shrink contracts around the (larger) bend in the hairs it shrinks on either side of the bulge and locks it in place better than otherwise. Because of this you don't need to glue the hairs in place, and there is no glue present to stop the cyanoacrylate from flowing through the bristles of the brush head you've created.

Using a pair of fine scissors, trim the brush head to the length and shape you require. Also trim away any sewing thread.

Step 3: Add the Brush Applicator to the Bottle and Use

You have created a pipeway that will channel the glue from the bottle to the bristles and 'wet' the brush head so that the glue can be 'painted' onto a surface accurately and with consistent thickness.
   For the ideal fit, slide the open end of the heat shrink onto the bottle's tip and heat the area over the tip to shrink it perfectly to the shape of the nozzle. Also, to minimize waste in the applicator, try to minimize any distance between the tip of the bottle's nozzle and the bristles of the brush head.

Invert the bottle and squeeze gently. It may take some time for the brush head to 'wet'. But only do this when you are ready to work with the applicator, since the glue on the brush will begin to set when it is exposed to the air. For this reason these brush applicators are single-use only.
     Squeezing too hard could send the brush applicator shooting off ot the bottle's nozzle (and messing CA glue). You shouldn't need to put THAT much pressure on the bottle. Perhaps even glue the applicator to the nozzle (temporarily).
     The same applicator can be used on syringes when they are the method of dispensing the CA glue. Heat shrink with a diameter of 4 mm should be used for a brush that is meant for use with a syringe. I have an instructable on using syringes for CA glue storage and dispensing - https://www.instructables.com/id/Cyanoacrylate-CA-glue-storage-and-dispensing-usi/

Step 4: A Possible Tweak to the Method

It's possible to use an intermediary piece of narrowing tubing to separate the glue bottle's nozzle from the heat shrink brush. This is handy when you don't want to get the little bit of glue on the nozzle itself, for instance when you're gong to need to exchange the brush a few times during use. It can also be used to match a smaller brush to a larger bottle's nozzle. It's not a necessary inclusion. The piece of intermediary pipe with reducing diameter can be made by heating plastic tubing and pulling the ends apart, then cutting to suite.