Are you looking for specific advice ? generally speaking:
- using the RGB palette is less distracting as you don't need millions of colours for flags.
- when you open a new image, make it 300 DPI and at least 3 time larger in terms of pixel than what the end image you need will be (ex: if you want the end result to be 400x600, make your work in progress image 1200x1800 then downscale it when you save a copy of it). As photoshop makes raster images, you can save as something smaller but if you save it as a larger image, it will get pixelated.
- save images as PNG, never as JPG otherwise you will end up with visible compression artefacts on large area of a single colour.
- when making an image, keep every distinct element in a separate layer. Its easier to modified one element that way later down the road. If you have a large number of elements that are part of a whole (ex: a coat of arms). group them together so it is easier to navigate through them and so that you can modified all of them as the same time if you need (ex: resizing)
- related to the previous entry, if you have a lot of elements, rename each layer so you know what it is at a glance. "vertical band-blue" is easier to remember than "layer 36".
- If you are just modifying the design of an existing flag, you can probably find a version of it on wikipedia. Although most flags are displayed as vector SVG images, if you go the the image page itself, their should be options to save it to your computer as a PNG image of varying size (always pick the largest one). Try to avoid taking non-open source images as people might taken offence to it if they find out and issue a take-down notice (I do)