How do I control the layout of custom fields?

You can control the layout of custom fields using the "Layout" option in the popup dialog for a custom field:

By default, the custom fields on an object appear in a single column, in the order that they have in the project popup, like this:

If you set the "Layout" option to "Start new column" on a custom field, this will start a new column in the layout, and place that field and the subsequent fields there. In the example above, if you set "Example field 3" to "Start new column", the fields will appear like this:

Note that the number of columns that fit horizontally will vary. If there is not enough room for another column, a new row will be started in the layout, and the column will be placed there.

If you set the "Layout" option to "Start new row", this will start a new row in the layout. In the example, if you add a field named "Example field 5" and set it to "Start new row", the layout will be like this:

Finally, if you set the "Layout" option to "Start new section", this will start a new section in the layout, with a horizontal line separating it from the previous section. In the example, if you add a field named "Example field 6" and set it to "Start new section", the layout will look like this: