Here are two very simple techniques that could help you instantly improve your composition skills in art. These are not really rules, but more like guidelines to help you with designing a more pleasing and eye-catching composition.
The Rule Of Thirds
Here are two paintings by artist Edmund Dulac to demonstrate the rule of thirds. You can divide your canvas into three sections vertically and horizontally, and the idea is to have the main subject of your painting along these lines or at their crossing point. You can see that Edmund Dulac uses this technique in many of his illustrations, and that is one reason why his paintings are always so interesting to look at. He also uses empty space in his compositions very effectively.
The Golden Ratio
The second 'rule' is the Golden Ratio. See how I've chosen a very simple composition for this portrait of a parrot, but I still made sure that the focal point of the portrait was the parrot's eye. That is why I used the golden ratio to help me position his face to the perfect place in proportion to the other elements in the painting. The Golden Ratio was used by master painters especially during the Renaissance era. The Golden Ratio can also be found in designs in nature, that is why it's also called the divine proportion.
I hope these two techniques will help you in designing the composition for your next painting. Thank you for reading!