Is there a good LEGO instructions manual creator for Ubuntu? I can't seem to find one.
Also, are there any online ones?
In case you have made your model with LDraw based software with all necessary steps already you can use LPub3d to create building instructions. There is a bit of a learning curve to make your instructions look right with required features. But then it makes them look almost like made by LEGO.
There is even an Ubuntu native *.deb package available to download. No need to compile from source.