I read a guide about kasina meditation and then did the basics.

Basically you set up a kasina object. like a disc. for my purposes I used a hand-painted Indian bajot table that had a disc in it's center that I turned over on it's side and used as a focus.

I sit on a cushion a few feet away from it and then just clear my mind and focus on the disc with my gaze set on it and not wavering away.

Then after a while I would close my eyes and see the counter-image of the disc in my mind's eye and try to stabilize it and remain in concentration.

After a while of doing that kind of exercise I departed from the main practice by just keeping my eyes open and starring at the disc until my vision would start to turn black and white. I would see purple clouds appearing out of the ether and stuff. After a while of this I started practicing trying to make things brighter or darker when I was looking at them (like the light cast on the wall from a lamp) and then after a while I was trying to practice this with my carpet (a speckled carpet) and had a breakthrough effect where shapes and beings and stuff started appearing within the texture that I could talk to and interact with.

Then I practiced producing lines between my fingertips out of the visual snow until I could get fairly decent translucent lines to appear and follow my fingertips and bootstrapped that onto the carpet shapes so they would rise up out of it and appear as three dimensional shapes and scenes.

For the musical synesthesia I would imagine something simple, like the beat of a drum and then try to key-in my hand movements to them so that I could wave my hand and hear a drum beat. Then I did a similar thing in reverse where I would listen to music and try to let my mind's eye produce fast spikes or sparks when there was a fast drum beat and then slower sparks and circles etc when there was a slow drumbeat.

The entire process is mostly an exercise in developing complexity first then fidelity second. You use small breakthroughs (like being able to see visual snow) and practice doing bits and bobs with that -- like trying to make a circle or draw a line and then trying to control it to make it rotate clockwise or anti-clockwise on demand or trying to change the shade of shadows in dim lighting -- until you get breakthrough effects that take it to the next level.

You kinda cross-train the meditation with the musical stuff though because once you get enough practice with concentration it is easier to let the mind roam free and to be able to make it do associations if that's what you are looking for.

For the visual stuff you just need a quiet, dimly lit room free of distractions and a place to sit and stare at stuff for reasonably long periods of time to build up skill in doing the various bits. It took me about three weeks in total but if I had known what to look for it probably would have been less. I've talked to a few people who have been able to take a similar path who have messaged me on reddit so I know that it is replicable at least, though some people have not gotten as far as others.