Your needs may vary slightly, but as a starting point that will PROBABLY work:
1 - buy a flip-up rear sight. These are designed to line up with your existing front sight. Put on the back of your picatinny rail.
2 - buy a 0.75" riser. Note, the height depends greatly on the red dot design. Risers come in many heights and are pretty cheap. You may need to experiment.
3 - put red dot on riser. Put riser on picatinny rail forward of rear sight.
A - to use red dot: leave rear sight in the down position, look through red dot, dot and front sight post should line up.
B - to use iron sights in a pinch, for example if red dot battery dies: flip rear sight to the up position, use iron sights looking through red dot as if it isn't there.
C - to use iron sights deliberately: remove riser holding red dot and set it aside until you want it again.