If you can control your line and always have your bowls return to the centre line then you are half way to being a competent bowler. The following exercise will help you focus on finding consistency.
1. Set up your rink with two bowls a mat width either side of the centre line at both ends of the green.
2. Bowl the four bowls and count how many are between those virtual tram lines.
2. Bowl the four bowls and count how many are between those virtual tram lines.
When you are regularly getting all four bowls between the tram lines you can move them in to be just a mat width either side of the centre line.