ClassiCube

The texture reference sheet by pressing F10 is generated from the current terrain textures, but I assume you're referring to making new textures.

In the folder where you put the game, there should be a texpacks directory in it. Go into that folder. Make a copy of default.zip, then open that zip copy.

Open terrain.png with an image editor (preferably other than microsoft paint, because that ruins the transparency of textures when saved), make the changes you want, then save it.

Start the game in singleplayer, press escape, then click change texture pack, then select your copy of default.zip to see your changed textures.

If you want others to see the textures, copy terrain.png out of the zip, then upload it to somewhere like imgur.

Then you need to get the direct URL of the image - in chrome, just right click the image, then click 'copy image address' to get it.

Then do /texture global [url] to change your server's textures to use the uploaded image instead.

Ok thanks for your helpful response!