How would I make a decal for a 2x2 dome brick or a 2x2 round slope brick?
The problem of creating a flat sheet that could be fitted onto a (semi)spherical surface (a dome head) is known to be impossible to solve perfectly, but there are viable approximations of varying precision. This matehmatical paper details one such approximation (for spheres, but the process can be modified for domes) using petal shaped pieces. By varying the number (and so the width) of the petals the approximation gets closer to ideal, but even with three petals a reasonable quality can be reached. Of course, much trial-and-error can be expected.

AFAIK The LEGO Group uses pad printing, this can be quite expensive to set up but produces the best quality prints.

For the R5 heads the process is much easier since the surface of this head piece can be flattened into a shape called an Annular Sector. With some measuring and calculation (and maybe some trial-and-error) it should be possible to design a sticker that fits this kind of head.