Every written language needs rules to keep things readable. Hieroglyphs had their own conventions to keep writings consistent and understandable.
Consistent reading direction is essential to readability.
English is always read left to right:
Hieroglyphs could be read either direction:
Reading direction was determined by human and animal hieroglyphs that appear in the writing. With hieroglyphic writings, hieroglyphs showing human and animals (and parts of humans and animals) all face the same direction.
The direction that hieroglyphic figures face dictates the reading sequence for that group of hieroglyphs. Reading starts from the direction that the characters face and proceeds towards those characters
Most commonly reading direction was from right to left. In this case, figures were right-facing. When human/animal characters are left-facing, reading direction is left to right.
Hieroglyphs are varying heights and widths. They can be nested together to reduce gaps. Shorter characters can be placed above one another to better fill the space. In this case, characters above are read before those below.
See the example:
Rows and Columns
Characters could be written in rows or columns, across the page or down the page. Rows and columns are separated by lines. Regardless of whether hieroglyphs are written in rows or columns, the reading order rules apply; reading direction starts in the direction animals and humans face, and hieroglyphs above are read first, then those below.