Simple Example (centering a single element)

HTML

<div class="aligner">
  <div class="aligner-item">…</div>
</div>

CSS

.aligner {
  display: flex; 
  align-items: center;
  justify-content: center;
}

.aligner-item {
  max-width: 50%; /*for demo. Use actual width instead.*/
}

Here is a demo.

Reasoning

Property | Value | Description | —————– | –––– | ———– |align-items | center | This centers the elements along the axis other than the one specified by flex-direction, i.e., vertical centering for a horizontal flexbox and horizontal centering for a vertical flexbox. |justify-content | center | This centers the elements along the axis specified by flex-direction. I.e., for a horizontal (flex-direction: row) flexbox, this centers horizontally, and for a vertical flexbox (flex-direction: column) flexbox, this centers vertically) |

Individual Property Examples

All of the below styles are applied onto this simple layout:

<div id="container">
  <div></div>
  <div></div>
  <div></div>
</div>

where #container is the flex-box.

Example: justify-content: center on a horizontal flexbox

CSS:

div#container {
  display: flex;
  flex-direction: row;
  justify-content: center;
}

Outcome:

horizontal centering