Open a new game, match or session. If gnubg already rolls the dice, don't bother. Press `edit', and either type the position_id and press Enter on your keyboard, or use the mouse to set up the checkers.
A mouse click at the border of the board empties the point. A left or right click sets up a certain amount of checkers depending on where exactly on the pip you click. Set up the cube with a right mouse click on it.
You can also drag and drop checkers when holding down the Ctrl key.
Finally, go to the menu and select `Game, Set turn' to choose the player on roll. The dice will then disappear.