Without repeating all existing track making guides let's just want to pick out some special methods. Please request things of interest in the Forum or by Comment.
You will need a Bitmap Editor to do the things explained here (e.g. MS Paint, Paint Shop Pro etc.).

How to paint something in an external program and import it to the track editor

The first thing you will need are the templates for the track and the according hmap (to draw elevations). Find those two template files here (rightclick to download). The track .bmp file has a size of 512x512 and a fixed color palette, meaning it just contains the colors used in GeneRally. Open the Blank.bmp file in your paint program, choose save as and name it with your track title (e.g. myrace.bmp). Also the hmap file has a fixed color palette (only greyscales, the lighter the color, the higher the elevation) with a size of 64x64. Open the Blank hmap.bmp file with your program, choose save as and name it with your track title and the word hmap (e.g. myrace hmap.bmp). You can now paint your track.

Choose the track .bmp file (myrace.bmp) and click one of the possible colors of the color palette to choose the color for your paintbrush / tool. You could for example start by choosing the green color and paint the whole picture in green to make it a grass landscape. Then you choose the dark grey to paint the track surface etc. By using the template, all RGB color values are chosen automatically. When you are finished with your track, you should open the GeneRally track editor, choose import land and check your .bmp file in. The hmap works quite similar. You should choose a dark color (best is black) and paint the whole file with this, then you have got a flat underground. To draw hills use ascending lighter colors (the template has two circled elevations as an example). Import your elevations to GeneRally by choosing import hmap in the editor.

How to paint extra cars like the ones used at Ace X? (requested by RCT2 maniac)

At Ace X and some other tracks here at Gentleman's, you can find additional objects like cars, trains, buildings etc. which are not provided in the GeneRally editor. These things are either "painted" or assembled by standard editor objects. The best way to do objects is to combine paintings and assemblies, this way you may "hit" these objects with your car, otherwise you would drive over them just like over a painting at the floor. Take a look at the picture at the left side to see an example of the original cars and some fake cars which are painted / assembled to the track. From the distant view point of the game, these objects look like real cars, though they are just simple assemblies at closer range.

If you take a closer look at the parked vehicles, you can see the assembled objects. The cars are done like this: At first, you should think about the layout of the object. If you would like to do a parked car, you should decide whether it should be a truck or a small car. To do a small car, paint some "wheels" to the floor (see picture top left edge) by making two dark bars (use the color for oil). Above this you can place the standard editor objects in the editor. For example the red and white striped car at the upper left side of the picture is made of two concrete walls (white part) and a soft wall (red part / green object in editor). You should always be aware of the perspective. If you have got an extremely low view angle at your track properties, you must place the objects a little more southward of your painting.