When I was first learning shoulder in I found that if I positioned the horse as if I was riding across the diagonal was a very useful guide. Remember to keep your inside leg on the girth and the outside leg slightly behind. Your inside hand should be soft and your inside shoulder back.
If possible have someone video you or ride facing a mirror. It will help you identify when you have the right angle and when you lose the angle. It also helps to ride shoulder in followed by a ten meter circle back to shoulder in. It breaks up the exercise and the circle helps you create the bend before going to the shoulder in.
First of all you need to know that the angle is between 30 and 45° depending on the horse.
To make it easier, you can start on a circle to help yourself with bending (with your interior hand).
You should use the same aids as to do a bend: your interior leg is on the girth, the exterior one is slightly behind the girth.
The exterior leg gives the impulsion and can be used as a "wall" that the horse can not cross.
The exterior hand is the most important to regulate your angle and direction but you have to start acting with your interior aids.
To help your horse, you can rise your interior hand a little and push your horse with your interior leg intermittently.
I hope i could help you, good luck for your test :)