Follow this recipe for a brushable bookbinding glue mixture.
Mix one Tablespoon of Lineco's Methyl Cellulose powder with water. It'll be clumpy. Stir a little and let sit overnight. In the morning it'll be mixable enough to be smooth.
If you're using Lineco's Neutral PH Adhesive, use less MC.
Try this ratio: 3.5 parts Neutral PH Adhesive to .5 part Methyl Cellulose mixture.
As you're working, your glue mixture will be continually exposed to air and will get thick and clumpy. Simply add a little of the MC mixture to thin it out again. This works both ways - if your glue is too runny, add more Sobo back in.
My favorite glue containers are the 2 cup Ziploc containers with a blue threaded lid. I put a little square of wax paper in between the lid and base as I close it. That way it's easier to open, when you're ready to work again.
And that's it! I've tried Elmer's glue, but it really isn't as nice. If you're en route and need an easy no-mess replacement, use Acrylic Gel Medium. It works great as a low-moisture adhesive, BUT will dry very quickly on your brush. Keep an eye on it as you're working.