Here's a simple tutorial for adding diagonal patterns on your vehicles. I started by looking at the GW paint job, and saw that one yellow triangle fit on the corner engine piece, or about 7mm. I then used a pencil and ruler to draw in the triangles.
More pencil work.
Then I started painting the yellow. I used the yellow base color, Iyanden yellow (now Averland sunset). The most important thing about painting straight lines is paint consistency. You want it watery enough to flow well, but thick enough to lay some color down.
You can see on these pics I magnetized the turret. That is a piece of thin sheet metal across the hole for the turret. The magnet is on the underside of the turret.
Then I went back and added in the black. I still need to touch up a few spots, but am mostly done. Pretty straighforward.