The "gallery rule" is to display artwork's focal point 57 inches from the floor.
The focal point can be a person's eyes for portraits, the horizon for landscapes or the artwork's center for all others.
Why 57 inches (4 feet and 8 inches)? This roughly corresponds to an "eye-level" for most people.
One side note: artworks that are positioned too high are uncomfortable for us to look at. So it's better to hang lower than higher.
WHERE TO HANG IN THE ROOM
Sun Exposure. Watercolors, photographs and works on paper do NOT like sun and will fade quickly. Oil paintings and acrylics also fade in direct sunlight, but less so than works on paper.
Humidity. Most paintings will not do well in areas with high humidity or water exposure. With some exceptions:
For bathrooms, aluminum panels or prints are ideal as aluminum doesn't corrode.
For kitchens, paintings with an external protective layer (e.g. glass, acrylic) would do well.
space around artwork
Filling the void
How should artwork match with other items in your room?
A good spot for a painting will fill a void in your room. It will not feel crowded or too busy. But it will also leave enough visual space around it.
For example, 6 inches is usually enough space between large furniture pieces and your artwork.
Notice the "halo" space around the artwork in the examples below:
If your painting already has a hanging wire attached on the back, please skip to the next section...
Otherwise, we'll need to get the following from Amazon ($10-$15 in total):
Then, carefully attach the D-rings approximately 1/3 or less below the top of the painting.
And attach the wire on both sides
Next step, nails!
Yet, I'd discourage from using regular nails or screws
Instead, these OOK Picture Hanging Hooks are my favorite
They have a very strong nail
They only make small holes in the walls
They look professional on a wall with brass nail support.
They come in a variety of different sizes supporting different artwork weights
the simple way
Instead of getting individual components like I mentioned above, an alternative is to get everything you need in a nice kit
I recommend one of these two all-in-one Hangman kits:
I haven't tried these kits myself but from the looks of it
They are the better option than buying individual components on Amazon