Are you painting 25mm, or smaller, or larger? I paint 98% 25/28mm, so my comments are based on this scale, for starters.
Here's my Quik-N-Ezee primer: tartans are essentially a pattern of stripes at right angles to each other, overlaid on a base color. So all you're painting, in this scale, are a series of stripes over a base color, building the tartan up from the bottom. For most minis, at this scale or smaller, three colors will suffice to paint an acceptable tartan (and most historical military ones). This is a cheat, but a trompe l'oeil that works on the tabletop or for display.
A good tartan reference book will help, but I suppose most of this info is found on the web somewhere these days. (But I'm a grognard and I love my painstakingly acquired library.)
Let's take a kilt for the Black Watch regiment, the "Government" tartan. A very common tartan. It's mainly blue, green, and black, in dark shades. Paint the kilt dark blue over all. Then paint horizontal and vertical medium to dark green stripes, medium wide, say an 0 size brush width, over the kilt. Keep the stripes uniform and equal distance apart. Start with the horizontal stripes and then paint vertical stripes of the same proportions. This should produce two to three horizontal stripes from the near-bottom of the kilt to where it disappears under a shirt or jacket. If you're really ambitious, or for a larger scale figure, you could add lighter green squares now to the places where the green stripes intersect with each other. Now, using a thin brush, paint a thin black stripe OVER every green stripe. Le voila! A very nice representation of the Black Watch tartan. Substitute a yellow stripe for the black stripe, and you'll have the Gordon tartan (92nd Gordon Highlanders).
Here are some WWI-WWII era Black Watch soldiers painted in this way:
(Note the piper in Royal Stuart tartan, a regimental distinction; this tartan is built up in the same way, just different colors and different pattern of striping to recreate the actual tartan -- in this case, scarlet paint and then dark blue pairs of medium-size stripes over all; then alternating yellow and white thin overstripes over each blue stripe; and a thin white overstripe BETWEEN each pair of blue stripes.)
All you need to do is stay consistent with your pattern, since almost all tartans are regular proportions. Add stripes of varying thickness to a base color, then overstripes on or crossing the initial striping pattern. You can built any military tartan this way, plus most clan tartans or invented tartans of your own devising.
Diced or tartan hose are similar, except you paint the stripes (usually red over white or black or green over red) at an angle, as are old-style Highland trews (trews in the British military do not have the tartan at an angle).
Here's some examples of Jacobite-period clansmen: link
Keep working at it and it will come easily to you.
