To Order (organize) the appearance of Custom fields on the booking form you will have to arrange them at the back end in the order you want them displayed. 

First you will need to unlock Ordering by clicking on the 2 arrows (pointing up and down) at the top of the first column in Custom fields list view.

You will see the rows of 3 dots in front of each custom field turn black (activate).

With the left button on your mouse grab the 3 dots (handle) and move the fields around in the order you want them to appear.