Although the Eaton Fuller 18 spd has 18 different ratios you rarely use them all. The are mostly designed for heavy loads or off road use so you have the maximum amount of gears to choose from. The nice thing is that if you do use all the gears the rpm drop between gears is very low like 200 rpm so the engine is always in its peak power range.

So far in game its pretty realistic how you can skip gears.

Here is a typical shift pattern I use with a light load from stop to top gear.

Low range
2L
3L
4L

Range shift
5L

From here you often need to start using the splitter (H-L)
6L
6H
7L
7H
8L
8H

So you can see that out of the 18 speeds I only used 10

In almost all cases you can just use the L (or direct) gear in the bottom range and drive it like a 13 spd i.e. 1L 2L 3L 4L

then range shift and go from there.

You rarely use the LO shifter position unless pulling away with a heavy load on an uphill.

Downshifting
As you see by the picture below the manufaturer recommends that you don't use H-L (splitter) when downshifting, just flick to 8L then go whole gears from there.

In real life you often wouldn't have time to get through all the gears anyway and it also keeps the engine egaged for longer periods so you can take advantage of the engine brake.

An exception would be if you think you only need to split it to get up a hill etc. or if you are trying to maintain speed not lose it.