On the first screen (settings) you have to give it a name and select a rollsign folder.
To make the digital ones, go to destinations, click + and give it a name and IBIS code. In IBIS 1, put what you want to come up on the display inside the cab, on ANNAX(1), put what you want it to say on the first line, ANNAX (2) is what you want to put on the 2nd line (if you don't have anything in ANNAX 2, the first line will fill the text area on the bus. ANNAX side is what comes up on the side of the bus. For RLB texture, click the folder, click Rollband_SD79 and select Blanko.tga.
For the other type, you will need to go to OMSI 2\Vehicles\Anzeigen and select a folder (for example B9TL_London) and open one of the textures inside in any photo editing software (photoshop or paint.net are good ones ). (this is so you know how big it needs to be) Put a box over the writing that's the same colour as the background and select the type tool. Write and position the destination and the number (and letter) and click save as. Make a new folder for your map and save it in there. Open Hof Suite and make a new destination. Give it a name and an IBIS code. In IBIS 1, put what you want to come up on the screen, in ANNAX (1), put the destination, leave ANNAX (2) blank, in ANNAX side, put the destination. Select the folder for RLB texture and find the destination blind you just made.
This is how I do it, there is probably an easier way to do it that I don't know of, but this way seems to work fine for me