Generally, the turn fins should be mounted as close to the sides of the boat as possible at 90 degree to the bottom. As the boat leans into a turn, the turn fin enters the water and acts sort of like a brake in the sideways direction. Note that they should “touch” the water only during turns!
Trim tabs aim to provide lift in order to compensate for changes in speed, weight distribution, and water conditions. When deflected downward, the water force on the trim tab will produce an upward pressure, thus raising the stern and reducing hull resistance. In many cases the increase in drag from the tabs should be more than offset by the lift provided by the tabs.
Generally speaking, the farther out the tabs are from the hull centerline, the more effectively they can function.
What is shown in this video is a FT010 with a pair of plastic shark fins installed. With such an upgrade the boat has a more stable straight line performance, and no more sudden sharp turn. Turning is not as easy as before though.
3d print turn fins can be viable and practical too!
This short video shows what is going to happen on a high speed boat that has its turn fins removed. As you can see, the turn is just way too sharp (and unpredictable) here.