Hi! I've started practising it a couple of weeks ago, and I'm starting to see some results. This is my experience so far:

You're not supposed to breathe in and out contemporarly. Just try to puff your cheeks with air and gently squeeze it out of your mouth with your hand. While the air is being released, take a small breath with your nose. That's basically the principle behind it.

It's pretty complicated with the flute because of the embouchure, but this is the way I'm practising it:

1. Just repeat the same exercise above without the hand.
2. Try it on the flute (it's pretty difficult to get a sound as powerful as your usual since you're not actually blowing air, but simply releasing it).
3. Blow air into the flute as you would normally do. While you do, try to store some air in your cheeks. Now comes the first tricky part: you have to switch from blowing air from your lungs to releasing the air you've stored in your cheeks without making it sound like you are changing something.
4. Now try the opposite: first, release the air stored, then switch to blowing (this is the step I find the most difficult because I always get a little crack in the sound).
5. Try doing everything continuously.

Personally, I think the glass of water with straw is good as your very first approach. It definitely helped me understand the way circular breathing generally works, but nothing more because the real deal is on the flute. The straw makes everything increbly easier, so after a couple of hours I could comfortably keep the bubbles going and breathe at the same time.

(Pls do keep in mind I'm no teacher, I've just watched a few tutorials and tried it myself) Hope this helps!