But on a more serious note. Examine the tartan you want to paint. See which colour is the base colour, and paint the whole area that colour, then pick the broadest check and paint stripes of that in both directions. And so on, but bear in mind that you may not manage all the colours, so just pick the most important - better to do a good job of a few colours than a poor job of lots!
And pick lighter/brighter colours than reality - it'll look better.
As Phil says, plan your pattern and familiarise yourself with it. I recommend having a test run with the paint on a piece of paper first. It will really help you to understand what you're you're going to do on the model and means you aren't starting cold.
Go slow and don't look at the pattern as a whole, it'll make your brain hurt.Think of it as basically two sets of check, one after the other. Each set of check is just a set of vertical lines, then a set of horizontal lines.
There's no such thing as a shortcut if you want it to look half decent. Take it slow, deal with each step in isolation.
Have a leather belt on standby to gnaw at if it all gets too much.
"You're a big man, but you're in bad shape. With me, it's a full time job." – Lt. Bromhead to Prince Dabulamanzi before the Battle of Rorke's Drift.
The Black Watch are the way to go for "first tartan". Base green. Add different green. Add blue. Job's a good 'un!
I painted Black Watch, Cameron, and Gordon Highlanders from Victrix, and the Cameron were the toughest.
There can be no doubt that the success of the attack on and stand against the enemy at St. Lambert sur Dives can largely be attributed to this officer’s coolness ... London Gazette, no.36812, 27 November 1944
A tip i have found very useful is before the final highlight, add a wash of equal amounts of GW Thraka Green, Asurmen Blue and water, it tones everything down and ties the colors together, hope this helps.
"This lighthouse is under attack - and by morning we could all be dead" The 4th Doctor - (Tom Baker) - Horror of Fang Rock