Shield designs can be a work-of-art in themselves.
Certainly, they can provide a challenge for the painter of 1/72 figures.
Both 28mm & 15mm figures are well covered (pun intended) by companies such as Little Big Men Studios with a lot of wonderful transfer prints for the Ancient & Medieval periods.
1/72, not very much at all, which leaves painting sometimes intricate patterns by hand, if the figures comes with a shield not embossed in the plastic.
I am by no means a fantastic shield painter but I have a few tips that might prove useful.
Geometric patterns are the easiest but even more complex designs (such as a Greek hoplites' Medusa's Head pattern) can be attempted by first priming in white or grey then using a fine point black artline pen to draw the design.
This allows for mistakes as the lines will be painted over but it gives you the opportunity to plan & fit a design onto an actual shield.
I also sometimes use paint pens, which come in a variety of colours, to actually colour the shield in.It provides a steadier stroke than a brush.
One of the biggest hurdles are unit shields eg for Romans where every figure carries shields with the same pattern. This will highlight small discrepancies from figure to figure.
While there are some master painters out there who can paint a Madonna on a Pavise or paint the most intricate medieval Arab or Russian shield patterns, that's far from what I myself can achieve. So for me the most simple solution is, search shield decals on the web no matter the size (there are lots out there whithout watermarks), download the pictures, put them into any program where you can downscale them (Wordpad or something), print them. Try this until they fit your shields, cut them out and glue them to your shields. Simple but efficient. I'm doing this with flags all the time. Otherwise, stick with simple shield designs, like the Spartan Lambda :relaxed: