There is a waterline marked, so that should help with alignment. It can help if you clamp a length of strip across each form at the waterline mark and then eyeball along the row to see if they are all even and level. Then check the keel line... I use fishing line.

I use the same method as Et.
If the waterlines are in the same plane, the verticals will be too, once forms are side-to-side adjusted so the the keel line is straight from end-to-end . IMO there's no need for either to be perfectly horizontal or vertical, but it makes aligning the stems easier if they are.

Once those forms are aligned, you need to add the end forms with the stem profiles attached. Getting the stems straight and square to the waterlines is extremely important as you don't want a twist or hook at the ends of the boat.

One other 'tricky' thing to remember is that - using non-bevelled forms as most of us do - the 'face' of the form that is toward the end of the boat needs to be on the station locating line. At the widest point of the boat, the forms will be 'facing in different directions' and the form spacing will seem to be narrower. Sounds confusing; just remember that the form represents a line on the plans - this can be better explained by others here, or consult your books.