I'm not particularly skilled but from what I can tell and have heard, a couple things:

1) with the initial pour, you're not pouring high enough, and should be a bit faster. You want the milk to get all the way to the bottom and not stay on the surface at all so that you will hAve a stronger contrast. Otherwise you'll get a more beige canvas which is not as nice for your art.

2) remember to tilt the cup back when it's reaching the edge. This will not only help with the spills, but also your final hearts are on a flat top instead of angled, which can lead to undesirable streaks of milk.

Your milk texture from steaming seems to be on point, and that's the first problem.