I feel your pain, having been painting Scots and Highlanders and various Celts for as long as I've collected miniatures. And almost all in 25/28mm (with the occasional 54mm showcase figures).
Here are some basic tips I can pass on, based on my own experience. First, I always find trousers harder than kilts. Cheerful news, eh? But possibly not as bad as body plaids/great kilts
Painting replica tartan is all about lines, unfortunately. So while you may struggle initially, in time it will make you a better painter all round. Your eye and hand will become steadier and better trained.
Study some tartan references, books illustrating clan tartans or a similar resource. Note how tartans are woven to a pattern of repeating lines of varying widths and colors that all intersect at right angles? That's the way to paint them. One set of lines at a time. First going in one direction, then repeated at cross-angles.
With trousers, tartan can be oriented either vertically/horizontally, that is, straight up and down on the leg, or diagonally (as for most old-time Highland trews, or on tartan hose). If the material is cut on the bias, it will fit tighter on the body and the tartan will be seen at an angle. Most ancient Gauls seem to have favored louse trousers, so your tartans will be vertical/horizontal, like modern tartan trousers/trews. That is to say, the lines you paint will be like pinstripes, straight up and down the legs top to bottom, and the cross-stripes will be side to side, around the width of the trouser.
Begin by painting a base color all over the trousers. For these periods, any shade will do. Gauls might have worn subdued tartans or gaudier ones, we really don't know. They would have been limited only by the dyes available to them and personal preference. So suit yourself -- perhaps the richest dyes/colors could be reserved for the elite classes while the common mob wears duller, more tweedy tones, but the Romans and Greeks were always impressed by the Celts' colorful garb.
Over the base color, paint a pattern of stripes of the same size and with the same spacing, first one direction, then repeat in the other direction.This will give you a basic check pattern. Over this, you can overlay more sets of stripes in contrasting colors. I find that it is best to make any wider stripes early and build up to your thinner stripes ("overstripes" as they're called in the trade). You might paint overstripes between pairs of existing stripes or directly over thicker stripes. You might also paint a small square of a lighter or darker shade of the stripe color at the intersection where thicker stripes cross. (Because when tartan is woven, the intersection of stripes is where the fabric is all one color and not blended.) Usually, in this scale, you only need to paint one or two sets of stripes to create a very presentable tartan. But you can add more, as long as you don't begin to overpaint all your previous work. Less is often more.
Here's an example of how I paint a proper, "regimental" tartan for a military unit. Let's pick on the 92nd Gordon Highlanders. They wear the Gordon clan tartan, which I reproduce in 25mm in three steps. First, I paint the kilt (or whatever) all over in dark blue. When that is dry, I paint a pattern of medium-thick middle green stripes across the blue, spaced evenly; usually two or three horizontal stripes are all there is room for on a kilt. But that is a good proportion in this scale. Then, I paint a very thin yellow overstripe down the center of each green stripe. You don't want to cover up the green entirely. The final result is a very good reproduction of the Gordon tartan at a tiny scale. And that is a pattern you could use again and again with different colors. If I were painting the Black Watch tartan, I would do exactly the same only substitute a thin black overstripe for the yellow one. Or if I were painting the Mackenzie tartan worn by the 78th Seaforth Highlanders, again I would begin the same way, only the thin overstripe over the green stripes would be white and in between every other green stripe I would paint a thin overstripe of red, centered on the blue squares that separate the green stripes.
This may be hard to visualize but that's the best I can describe the process. Not everyone can paint good tartans but if you can manage it, it's a rare skill and will really make those Celts stand out. Take your time and paint clean stripes and the result will surprise you.