It amazes me how often this topic comes around and around.
To sum up what most people say:
Try your hand at Blender (don't do what I've done - avoid Sketchup)
Don't use paper buses as diagrams, they aren't always accurate
Search up online for videos on how to use Blender. There are plenty of them.
Don't be over ambitious.
Start with a basic shape like a rectangle, and once you're competent enough with the process of making it and getting it in-game, gradually build up to doing the bus. As in don't go straight into doing the whole bus first time, it isn't easy that way. (again, don't do what I've done)
Although of course it would be pretty cool to have a Bristol LH in OMSI. We could do with some older UK buses.