Type your desired text, then click the Download button in order to download the patch texture. After that, upload the image into Second Life, get the texture's UUID and set the texture using the script in the uniform or simply by using the build tools.
You can also get the patch files without text so you can edit them yourself in program of your choice: Black, Olive, Sand. If you need to see an example of edited patch, click here.
In order to apply the patch after upload, you need to go to the uniform's menu (see manual for details), select PATCHES and type the UUID of the freshly uploaded texture. See the picture below for reference: