The mixture will be quite thick and gloopy (see pics ^^^ in post). You may need to use your fingers or a slightly wet flexible spatula to spread it evenly into the mould holes, but DO NOT flatten it, or you'll lose the airiness.
Don't overbake. Expect the buns to rise and then deflate a little.
THIS is the correct yorkshire pudding tray size (U.S. option
HERE). Using a baking tin with wider diameter holes may prevent the buns from rising at all. Inversely, if the cavities are smaller in size, the buns will likely rise more, but you will need to bake a few extra minutes to ensure they're cooked through.
Store loosely covered on your kitchen worktop. No need to refrigerate. Can be frozen.
Read ^^^ post if you're intending to sub the fiberflour. Using 20g coarsley ground psyllium husk, I ended up with a very thick foam that I 'shaped' into the moulds. Baking time remains the same. Macros for psyllium version are in the post ^^^.
Metric kitchen scales are an inexpensive yet invaluable gadget to ensure accurate measurement of ingredients. Store them upright in a cupboard or over your worktop and they'll only take up a tiny bit of space. Click
HERE for the ones I use. For U.S. option click
HERE.