Really looks great. You had a lot of good ideas on this one. Love the little details like for example santas book or the bag with presents
Diluation work done... something like this? ;-)
Jap the freehand looks pretty good but the white on the teeth seems to be a little thick again.
But not as bad as on the cane. The color gets to thick when the paint itself becomes 3D ^^ On the cane you can really see little "paint hills". That's too much. If the paint doesn't cover over black, it's better to use several thinner coats.
Especially light colours with white mixed in tend to cover better then you would probably expect, even with just 2-3 thin coats
Much harder are saturated colours like red or especially yellow.
When you're having trouble to get coverage with yellow or red, mix some white to it to get a good base layer with a few coats. Over that coat you can then paint your yellow/red/ whatever.
Adding white works because 99% of acrylic whites that are sold (just my estimation) are titanium white which is a very opaque pigment.
As BaM already wrote, getting the "right" thinnig ratio is feeling. It highly depends on what paints you are using (within ranges even different tones need more or less).
So giving ratios like 1 drop paint with x drops water can be very misleading. From own experiance I can tell that a lot of beginners tend to use way to thin paints when they start thinning their paints ^^
When I joined the "Church Of Thin Paints" I followed such a recipie like a fanatic disciple and glazed on base layer over base layer for ages xD
Compared to that I use much thicker paint now.
So I think a better way to explain is to describe the properties.
The paint should flow easily from the brush to create even coats. They usually won't cover completely on the first go but you should see that you painted something on there. Most paints will probably cover with 2-3 layers.
It's probably best if you use an old brush to thin and mix your paints -> but I usually use my "paint brushes" -> yep I know it kills them ^^
The mixing routine itself isn't that scientific. I put some paint on my palatte, load my brush with water in the cup, dap it on an empty spot on te palatte to get rid of most of it and the mix the rest into the paint. If it's still to thick -> repeat
The consistency is usually like milk, maybe a bit thicker.
I then clean the brush. Wipe it, load it with paint. The important part then is to tuch your kitchen towel first to suck some of the liquid out of the brush. If your brush is to full you maybe flood the mini with too much paint that is hard to control.
So it's a bit try and error in the beginning but you'll probably get used to it pretty fast.
Creating transitions with layered glazes is another topic of its own

But one step after the other ^^