It all really depends on the bus,
Most commonly, you'll have templates for all the different textures of the bus; with most buses, you should be able to go to the texture folder of a vehicle and there should be a template folder, if not, it will be in the repaints folder in the texture folder. Then all you need to do is either use paint.net or photoshop - which I prefer - and set it so there is the template as a layer set as multiply and another layer beneath set as normal. Now all you need to do is add your wanted textures to that.
Once you've done that, you'll want to save it preferably as a PNG in the texture folder of the wanted vehicle (unless it's the C400, then either go in
Livery_R or
Livery_R_Express). Now go to the CTI file of the repaint you want to change the texture of, scroll down till you find a section titled '
[item]
Name of repaint
Seats
Name of Seat Texture File
'and change the name of the current PNG to whatever you've named the seat texture you made earlier.
Now note, if there is a file name before that, eg. '
Yellow Bus/-2017/seats.png' then just remove that block of
text. And if the CTI you want to add the texture to doesn't have whats in
red, just copy it in underneath the one listed '
[item]
Name of repaint
Exterior
Name of Reskin File
'and that should do it for some.
Now others like the E400MMC which I'll demonstrate with just have textures and no templates which you can't often change in repaints. So first, use this link HERE and copy whats in the text folder to 'Vehicles\ADL_E400MMC_NWFB_3800\Model\model_3800', and place it under the 'Texture Changes' section, so all the numbers of the changes are sequential. I'm not sure if you need to do this with others as I haven't been repainting busses for that long, but just keep your eye out. Now, for the E400MMC, go into the texture folder and open '3801_chair1' with paint.net, and simply add another texture over that. Save it under a different name in the 'Advert' folder, and open the CTI you want to change. Now add this with the repaint name and the name of the texture you made.
[item]
Repaint Name
farbschema_tex25
Texture Name
And that should do it.
NOTE1: The E400MMC has it so you can't change the texture of the lower seat cover on the 'disabled' seats - not backrest - but I'm currently looking into that and might update that file by the end of today.
NOTE2: The Eclipse 1.5 uses the same method but it doesn't have an 'Advert' folder - DON'T make one, just put it in the Repaints folder with the others, also, for the Eclipse, the one you'll want to edit for the seats is'
[item]
Name of repaint
farbschema_tex6
'
NOTE3: I'm also not sure if you can change any interior textures on the Olympian as I haven't seen it done before, and don't expect you'll be able to change it on every bus, but everything on fellowsfilm (except the Olympian) can have interior textures changed.
NOTE4: If you are changing the seat texture of a repaint you have got off of fellowsfilm, or pretty much all repaints you haven't created, you CAN'T upload them here or most other places due to copyright - which you probably know already.
Sorry this is long for what it is but I hope this at least helps.
And if you don't fully get that, then watch
THIS about the ALX400 by Citaro from around 6:59