Hanging art and decorations on walls is not tough, but it looks hard to a lot of people. You may fuss endlessly about "getting it perfectly right" but there is not only one right – usually plenty of options will work and you can always move things around later if you desire.
First decide what you want to hang and where. Many people make the mistake of putting too much stuff on their walls, so you may need to pick and choose. Hey, the most famous museums in the world often have lots of great paintings and other art in storage and rotate what they display. You can too.
For each piece, decide how high it will go on the wall. As a general rule, which can be greatly ignored if you'd like, pieces go about twelve inches over furniture like the back of a coach and otherwise in the middle of your line of sight, typically sixty to sixty five inches high. Of course your person taste also comes into play, for example my mother in law likes her art and other wall hangings significantly lower.
Smaller pieces are usually grouped together which creates a nice effect. They can be grouped vertically, horizontally, in a square / rectangle pattern, or however you prefer.
If working with someone else, have them hold the pieces on the wall so you can see what they look like. If by yourself you can cut out similar sized pieces of paper, for example newspaper, and tape them gently to the wall to see what the placement looks like.
Above all, remember your walls and art are dynamic. There is no wrong, and you can change them whenever you'd like. If needed, you'll find all kinds of hanging devices at any hardware store.