Watercolor can be so frustrating at times, so I’ve compiled a few tips.
Start with a good sketch.

I heard once that the better the sketch the better the painting will turn out. I like to sketch to plan out the composition, before drawing on the watercolor paper. Doing lots of erasing can hurt your paper before you start painting.
Mix enough color all at once.

Mix plenty of the pigment so you don’t run out of the color halfway through your painting. It can be hard to match the same color when you run out. When you have all the color mixed, it is easier to work quickly and get a nice even wash, which is easier when you can do a wash all at once (before portions of it dry).
Test out your colors.

Use a piece of scrap watercolor paper to see what the color looks like before putting it down permanently.
Create a reference color sheet.
Artists often make a swatch card of their watercolor pigments so they can see how their colors on paper will look (also to see how they dry-watercolor dries a bit lighter). Artists also create a reference guide to see how combinations of colors look as well. This is useful for a reference for color mixing and recreating colors. This way you can see how mixing a variety of colors will look like. This can make planning your composition easier.

Use heavy weight paper.
You will like the result better and it can take on more heavy washes. You can look at my post about watercolor papers for more information about this.
Start painting!
This is a just a few tips to start you off to feeling successful with watercolor. I hope you feel more confident and happy with your projects with these tips!
If you have questions about watercolor or supplies put it in the comments. Also, share what you’ve been working on and if the post was helpful!