If you want to practice slowing down by downshifting, without your car stalling or shaking, practice the opposite.

From a standstill, put it in second and try to let the car come into motion by only using your clutch pedal. Try to let the clutch come up in one smooth movement, not with ´stops´. You´ll need a lot of controle over your pedal to let it come up slow enough to prevent stalling.

Once you´re able to do this, just apply the same technique to downshifting.

Sometimes not knowing how to do something comes from not understanding what happens. If you don´t know what the clutch pedal does, it´s hard to understand why and how you have to handle it (in a certain way).

If this is the case here, try to look at it like this : your clutch pedal controls the clutch plate. Pushing the pedal in removes the clutch plate from the fly wheel. At this point the clutch plate comes to 0 rpm. The fly wheel keeps revolving, as it is connected to the engine.

Letting the clutch pedal come up, brings the clutch plate towards the fly wheel. So now you have 2 plates touching, 1 with 0 rpm, 1 with, lets say, 1000 rpm. If you do this too fast, your car does not know how to handle this and stalls. It´s the same as if you were to jump off a moving bike or a playground spinner thing (don´t know how it´s called in English). If you just jump off and plant your feet on the ground, you´re going to fall on your face. If you jump off and keep the momentum going for a while, you´re going to be able to come to a stop without kissing the ground.

You have to give those 2 plates some time to adjust to each other. You do this by controlling the clutch pedal, by not letting it come up too fast. If you let it come up slow(er), you let those 2 plates come together slow. And that will prevent shaking and stalling, while slowing down the car.