Well your question as posed is kinda hard to understand, lol.
how can I create a brand-new texture file for this new model?
In PSP (or whatever your paint program), click the File menu and select New, then specify an image size of 256x256 pixels, and that's basically it. Paint it and save it as you would if you were repainting a texture (only more so as you need to construct the details rather than just change colours over them).
Or, open an existing texture, optionally blank it out, save to a new name and again paint it and save as you would for a repaint.
I understand you want to paint the Dodge front with Grave Digger flames and artwork, so prolly what you mean (or what you think) is to combine the Rampage and Digger textures together?
- You could start a blank texture as above, then open both Digger and Rampage textures, and copy-and-paste the bits from each that you want to keep, i.e. copy the front from Rampage, and most the rest from Digger, and paste them into your new image. It may require some work to get everything to fit.
- Or, just open the Digger and Rampage textures, copy the front out of Rampage and paste it onto the Digger texture - again it may be tricky getting everything to fit, but you should basically be able to paste Rampage's nose over the space in the texture used for Digger's nose with maybe a bit of resizing. The advantage is you'll keep the existing layout of the Digger textures, so you won't have to remap as much of your model.
Then you'll return to BinEdit to swap the Digger and Rampage textures with your one new texture, and remap the front hood and probably the grille and bumper to the right areas of the texture.
- You could even just remap the hood to the appropriate area of the original Digger texture, and leave it at that without even making a new texture. There should be a clear edge of the faces between the truck grille and bumper, which you want to keep, and the hood and fenders which you want to change - so you can just remap that part without changing the nose (unless you wanted to paint green around the headlights and edge of the grille to make it match, instead of the current black). The downside of that though is that most of the model is mapped to the Digger texture and just a part of it to the Rampage texture. That's not as efficient as it could be as you usually want to minimise the number of textures used. Though as I recall the MTM1 trucks used only 1 texture any, and bumping it up to 2 is hardly being unmanageable (after all that's what the stock MTM2 trucks use). But that may be something to consider.
All the above assumes you're happy just applying the original Digger hood texture over the Dodge hood. Since they're different shapes it may not appear to fit. But since the truck is black with minimal painted body features it should probably look fine.
If you wanted to get really fancy, you get into scratch painting which is kinda also what answers your question (how to go from a blank canvas to something you can map onto your truck model), but I think is getting way too far along for what you want. But regardless, you may be ideas from the following resources:
A discussion on truck painting: http://forum.mtm2.com/viewtopic.php?t=2940
references the advanced tutorial from here: http://mtm2.com/~d2s/binedit/
where I made an entirely new model and then needed to create entirely new textures to map to it.
Cale's excellent PSP7 advanced paint tutorial: http://cownap.com/~mtmg/trucks/psp7.html
For what you want though, I would recommend you go with the number 2 process I outlined - make a copy of the Digger texture saved to a new name, and repaint the nose (presumably by copy-and-pasting from the Rampage texture, and colour changing) and apply it to your truck, remapping it to the Dodge hood and front as appropriate.
Hope that helps some, feels like I must have made it much more complicated...