The best practice for a bowler is out of the nets and in the middle bowling to a keeper with no batsman to worry about or balls to fetch. Just focus on learning your bowling skill through simple repetition and subtle technical and tactical improvements.
Encourage young spinners to spin the ball hard
1. Length is vital -- bowl full and drag the batsman forward
2. The batter should feel his stumps are under threat
3. Be ready for caught and bowled chances every ball.