Hehehe how I loved being a de facto bowler for the winter...
...things I learned. Firstly, its best to use the Sidearm from about 17-18 yards rather than from the full length of the wicket - I think this is because you use an action similar to throwing a ball, which involves bracing over the lead knee, which reduces your height and - if you bowl normally - your point of release, compared to where you mentally expect it to be.
Secondly, you can't deliver a full range of deliveries with an identical action. For the fuller stuff, its best to try and get the ball end of the Sidearm as high as possible, because this reduces the batsman's margin of comfort on the shot (ie, they can't just lean into the ball and get away with it, thereby ensuring productive practise as opposed to ego boosting). When pitching shorter, either as proper "chin music" or just to work on back foot technique, its best to use a slightly slingier action because this replicates the flight of a normal bouncer as opposed to steepling bounce.
A good action for learning to use the tool is to try and deliver a colts size ball (ie 4 3/4 oz) from a kneeling position - this will give you a feel for the force required to dislodge the ball from the cup with a controlled flight.