Optician used to tell me to work the muscle by following my finger to my nose, trying to maintain a single image. At a certain point it will snap into two - the 'lazy' eye has given up and drifted slightly - the goal is to get the finger as close as possible. Obviously if you get very close or all the way, that's 'cross-eyed', but I just can't do it.

Here is what worked for me. I used my laptop, zoomed in a bit on the images and brought the screen fairly close to my face. I ensured that the image was crisp using each eye (I also have astigmatism, and I probably also need reading glasses, but there is a sweet spot where both eyes have good focus, and I ensured I was there.) While crossing my eyes a bit, I start to see a third image in the center of the two images, but it's either out of focus (like two overlapping images), or it's very thin, like it's not the full image. I relax and keep my attention on this imperfect image and try to focus on it without trying too hard. Using this approach the image suddenly comes into focus and I no longer have to try to keep it there.

I feel like the key might be to notice the very beginning of the desired image in the center and then to try and focus on it, but in a bit of a relaxed way.

Incidentally when it works it is extremely weird! The other images essentially disappear and it's like you've travelled to another dimension.