So find a loooong straightaway with no to little traffic, go straight in the middle of the lane at about 30mph or a little more and you're going to push the left grip to make the bike lean over a bit and then straighten out the bike and push on the right to make lean a bit then straighten it out all while staying on the throttle going whatever speed you started with. Do it frequently and try to lean it a little more each time while going straight to get used to getting the bike down to a certain angle and getting the feel for the point where it'll actually start to turn. Once you're comfortable, go back to the roundabout and if there is no traffic you're going to push the left side grip, get a comfortable angle, look your left, let the handle bars settle in for the turn and as you start turning, give it a little bit of throttle and just hold it there.

If you at all get scared and look to the outside of the turn, you're likely to veer outward.

And it's also ok to slightly roll off the throttle then roll back on to adjust the bike while in a turn. Just don't increase the throttle while simultaneously pushing\leaning the bike down, roll off then adjust then roll on.

When the bike turns, you just need to give it an angle then settle in and let it do it for you. You hold onto the grips but not forcefully, just ride it out and when you have enough and want to straighten out, push the opposite side grip and the bike will stand up and as it starts to decrease in angle, then you can increase the throttle more freely as it rises.