Spritetool's readme explains how to make a sprites.txt:
Originally posted by Spritetool
The sprite list is a text file. It specifies what you want to put into your ROM. Each line consists of a sprite number followed by a sprite cfg file. An example is shown below:
How do you pick a sprite number for a custom sprite? Well, it depends on the kind of sprite it is:
00-BF Standard Sprites (sprites included in the 'sprites' subdirectory)
C0-CF Shooters (sprites included in the 'shooters' subdirectory)
D0-DF Generators (sprites included in the 'generators' subdirectory)
So, if you just want to insert a single sprite, make a new .txt file, and type "00 nameofyoursprite.cfg". Make sure the .asm and .cfg files are in the 'sprites' subdirectory, though.