Making your own texture is possible by using any drawing program (Paint, Gimp, Photoshop...).
But if you wish to upload them on your character and make them visible by the other players, you'll need to buy the texture items (Head Texture, Ground Texture etc...).
If you want to see the preview of how your textures will look like on your Tori, save them in your custom folder and save the textures with the different texture names.