Your best friends are vector lines (pen tool) selection and masking. It also helps to have a good grasp of layout tools like setting up grids, cropping and good folder layout. In fact, if you are patient enough there isn't a design I have seen on this website yet that you could not recreate without a drawing tablet.
The best tutorials for making flags are going to be how to use Photoshop in general. Here are some ones that might be a good start:
Learn about the different pen tools in Adobe Photoshop and how you can use them to draw lines, curves, shapes, and paths.
helpx.adobe.com
Several years ago a friend of mine asked me to teach him how masks work in Photoshop. This is my incredibly late response. We’ll go over the basics of what masks are, what they’re used for and how wielding them properly will take your Photoshop skills to an entirely new level. ...
designshack.net
Learn how to use guides and grids in Adobe Photoshop to precisely position and align images or elements. Smart Guides, which appear automatically when you need them, help you line up your shapes, slices, and selections.
helpx.adobe.com
The best way to get clean, crisp details is to make vector lines. Vectors calculate shapes based on a mathematical equation while rasters (the sort of stuff you would ordinarily make if you were 'drawing' in PS) assign values to pixels. Practically, this means that a vector line will always be crisp at any resolution while a raster will get blurry and messy if you zoom too much. You can make straight lines with rasters, but its a bitch. With vector tools you can crank out perfect straight lines, curves, shapes etc. left and right. Use vectors to draw, masking and selection to color and/or correct your designs, and fit it all inside a well-proportioned grid.