For years I relied on the internet to find information about ceramics. One of the most useful and reliable sources both then and now is Vince Pitelka’s website. There he shares his knowledge on all aspects of the studio, from making your own tools to loading & firing kilns. (Although I have yet to try his Fabulous Top-Secret Extra-Special Spicy Cheesy Black Bean Dip recipe!) https://www.vincepitelka.com/handoutsinformation/
One very useful recipe on his website is for Mason Underglaze, which actually includes three recipes: CMC solution (great on its own as a glaze additive to slow drying and improve brushability), Underglaze Liquid (which can be used with any underglaze to greatly improve brushability), and finally the Mason Underglaze recipe itself. If you do any underglaze decoration, even just to sign your name, I hope you’ll try out these recipes!
Mason Underglaze Recipe
One note: You should be able to purchase “food grade”, non-toxic propylene glycol via the internet, DO NOT just buy automobile anti-freeze which may contain other additives.