I like to buy cheap little polishes to mix with. That way if I don't like it I can toss or dump it out. These polishes are often under pigmented, but have a base color to start with.
I then chose my loose mineral powder pigments or make up, clear polish, glitters, and polish colors to mix in.
I use my cuticle scoop to add and mix the powders. They powders need to be mixed in well and I sometimes need to thin it with the clear polish. Add a little at a time until you get the color you desire. For a few colors I may have to add a drop of colored polish or even a glitter polish to get the look I want.
Today I started with a sheer peach pearl color and added a pinch of dark brown pigment, a shimmery rose glow powder, a drop of red, 4 drops of clear, and 6 drops of pink glitter polish. Some loose glitters can melt and clump up.
These are more appropriate for fall now. A rosy nude with subtle glitter, an antique gold, a dark burgundy brown, a dark purplish burgundy, and a semi translucent bronze with gold and brown glitter.
This last one was made for adding a darker gradient to the tips. I really liked it paired with this silver gray here.
The burgundy colors were mixed because I did not have an oxblood polish. I like both in their own way. The antique gold was a happy accident found by mixing a sheer gold shimmer with olive eye shadow powder. I think it also looks great with a glitter over it.