Align the flex items at the center of the container:

div {
  display: flex;
  justify-content: center;
}

Align the flex items at the beginning of the container (this is default):

div {
  display: flex;
  justify-content: flex-start;
}

Align the flex items at the end of the container:

div {
  display: flex;
  justify-content: flex-end;
}

Display the flex items with space between the lines:

div {
  display: flex;
  justify-content: space-between;
}

Display the flex items with space before, between, and after the lines:

div {
  display: flex;
  justify-content: space-around;
}