Firstly you can download rider template HERE---->  [Please login to see the link]

You can use any photo editor program to customize these files. 

After that you have to export as JPG. Tag name must be:

For the 675R: YOURSTEAMID-skin-675r-livery

For the Scooter 70: YOURSTEAMID-skin-polini70-livery

For the Scooter 94: YOURSTEAMID-skin-polini94-livery

For the Ohvale 110: YOURSTEAMID-skin-ohvale110-livery 

For the Ohvale 160: YOURSTEAMID-skin-ohvale160-livery 

For the Ohvale 190: YOURSTEAMID-skin-ohvale190-livery 

For the Ohvale 212: YOURSTEAMID-skin-ohvale212-livery 

For the YCF 160: YOURSTEAMID-skin-ycf160-livery

For the YCF 190: YOURSTEAMID-skin-ycf190-livery

For the 1000pro: YOURSTEAMID-skin-1000pro-livery

For the 1200pro: YOURSTEAMID-skin-1200pro-livery

For the bagger: YOURSTEAMID-skin-bagger-livery

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You can also edit the material of the skin. You have to export as TGA.

Edit tips:

Red: metallic

green: occlusion

Blue: full black/full withe

Alpha: specular

Tag name must be:

For the 675R: YOURSTEAMID-skin-675r-coat

For the Scooter 70: YOURSTEAMID-skin-polini70-coat

For the Scooter 94: YOURSTEAMID-skin-polini94-coat

For the Ohvale 110: YOURSTEAMID-skin-ohvale110-coat

For the Ohvale 160: YOURSTEAMID-skin-ohvale160-coat

For the Ohvale 190: YOURSTEAMID-skin-ohvale190-coat

For the Ohvale 212: YOURSTEAMID-skin-ohvale212-coat

For the YCF 160: YOURSTEAMID-skin-ycf160-coat

For the YCF 190: YOURSTEAMID-skin-ycf190-coat

For the 1000pro: YOURSTEAMID-skin-1000pro-coat

For the 1200pro: YOURSTEAMID-skin-1200pro-coat

You can see your steam ID with this online tool [Please login to see the link]

all the JPG files must be placed in folder ...Steam\steamapps\common\TrackDayR\Track Day R_Data\StreamingAssets\skins