I have not provided information about the exercises without prisms, that can easily be carried out by everyone, although I recommend you see a specialist.
One is the cat stereogram. After some practice I learnt first to maintain the three cats even after removing the pen, and next to make the three cats appear from scratch without using the pen at all. I also made the exercise more complicated by moving the sheet to the far left, then to the far right, up and down, and finally in circles.
I was also shown a variant of this exercise that does not contemplate the use of the pen at the start. You hold the sheet with the two cats at arm distance straight in front of you. You focus on a well defined object far away, for instance something 2-3 meters away at a similar height of the sheet. If you truly focus on that far away object which is on the same line of sight of the cats, the third cat will appear. You hold.
The cat stereogram can become fun. I can now see three well defined cats for as along as I want, like 3-5 minutes. Then I stop because it gets boring and stressful. One of the issues I encountered and I got better at is that sometimes one of the two outer cats would fade for a split second. I am not sure what this is due to, because the cat in the middle would stay, so it may not be simple suppression of one eye.
The other exercise is a sort of Brock string exercise but aimed at enhancing convergence at near. Therefore, no cord needed. Just a print out such as the one below (the length of the line is circa 20 cm).
The "nose" side is indicated. You keep the sheet inclined downwards, for instance, by 30 degrees, and you focus on the farthest away square. You do not jump immediately to the next square inwards. You first need to make sure that the square you are looking at is neatly defined and two lines depart from it towards your nose. You hold for a while, I typically do it for 15-30 sec. If you can see "the right pattern" of square and lines, then you move on to the next square. This time the line on the sheet will become a sort of X. You hold and try to control both the shape of the square and the X, and move on. Depending on where you are at, it may take you a while to reach the square near the NOSE label.
I was recommended to not overdo. 4 minutes a day are enough at the start.
I confess that at the beginning I was underwhelmed by the exercises I was given and I did not take them very seriously. I got more excited when I started using prisms and then when I noticed that I could take the pen out of the equation in the cat stereogram exercise. These exercises are also a way to find some relief in the presence of extreme eyestrain and neck tension.