There are many different ways to add ABS slurry through your printer bed each has their own advantages and disadvantages.
One of the easiest ways to get a full and like coating across the entire bed to use an atomiser spray bottle. Simply decant some acetone into the bottle at roughly 1/10 volume of old waste ABS scraps and give it a little shake.
Observe over the next hour and you will see the ABS fully disintegrate into the acetone.
You should note that white ABS sometimes contains other articles which do not dissolve, however the ABS slurry will contain plenty of ABS and the atomiser would do a good job of filtering those particles out.
Another method is to add roughly 1/2 of a teaspoon of acetone directly to the bed and to use a solid waste piece of ABS as a kind of park and to swirl acetone evenly over the bed.
One issue with this method is that it’s very difficult to achieve an even coverage and typically solution will build up in the centre of the bed.
Another method is to mix up ABS slurry in a jar and use a Q-Tip or paintbrush who directly paint onto the bed.
The main issue with this again is the thickness and evenness of distribution.
Looking for the glass to have a kind of frosted coating. You do not want an entire layers were of ABS directly on the bed. It won’t make it hold any better.
I aim for around two full coatings which is barely visible and wash the entire bed down with meat ABS between major prints and then begin to apply evenly again.
Remember you need to re-level your bed almost every print. You also need to ensure you are using a slurry the same colour as your printer ABS. Even if it’s not you may well be able to remove most of the colouring using a sander to take off that initial first layer.
What you are essentially creating is very thin yet wide solid raft for your model to be built upon.