If you don't know how to make a skin, just go to google, type in FXR LFS skin and choose a skin that is closest to default skin (i.e no custom graphics, just pure single color or white). This is mainly to avoid plagiarism of other people's intricate skins.
Then download paint.net, open that skin JPG/PNG file in the app and then download the NDR GT2 car template with the car number and windshield stuff. Resize to the skin dimensions using mouse drag and holding down Shift key.
If I recall Paint.net doesn't allow editing of Photoshop psd files hence you will have to color over the car number using the same red color of the number plate (use the Eyedropper tool on left panel), then just use text tool (T, also on left panel) to put in your number, I'm sure this will suffice.
Then save the file as FZR/XRR/FXR_yourskiname as JPG or PNG and upload it to LFS skin uploader, then upload the skin to this forum as an attachment.
If you are unable to do, just let me know and I'll create a blank car skin with your number for you, but you have to upload the skin yourself (don't have much skin slots remaining for myself).
I have put together a "default" NDR Skinpack for GT2 skin upload use - this will be useful for Drivers who are unable to find a skinner and are not able to edit or make their own skin.
Attached below are all skins for the 3 GTR cars in (slightly lighter) NDR ocean blue with the appropriate branding.
To make the skins race-able, please download them in either the JPG image format or the .psd format; .psd only if you have Photoshop.
If you do not have Photoshop (which is probably why this would interest you in the first place), download any of the following applications - Pixlr, PicsArt Mobile, Paint.net Desktop, Microsoft Paint or any suitable editing app. (Edit: Thank you DriverLT for suggesting an Online Photo Editor https://www.photopea.com , this works too . Load the JPG image and put a text-box with your approved Car Number in white over the GT2 Challenge Car identification decals (the red boxes). Then, upload the skin to LFS via "Car Skins" (under Account).
Please be reminded that this skin is free access. Please edit and upload the skin yourself. You could even download the image and edit it on your phone if necessary.