They used the method still used today, where planes fly in formation, at increasing heights.

So the first row of planes was at 5000ft, the second row is at 5100 feet, etc. Even with static lines deploying the parachutes out the side door, parachutes took a second or two to fully open and slow down the parachutist. So this delay helped them drop through the turbulence from their plane. And stopped them being lifted up, like a dandelion into the path of planes behind them. Being loaded down with weapon and ammunition also helped.

More importantly the height difference was relative. So if pilots got delayed/lost and found themselves behind and below another plane, either they would steer away to be at the front of their own column, or climb above the plane in front .

Stopwatch navigation, now automated by computers and GPS, was taught for this reason (also bombing). You still see it today in Football matches that have planes fly through a 3dimension point above the stadium, in a precise direction, to the timetabled second. In theory you can schedule a continual train of planes from different airfields overhead the point(s) of your choosing.

In practice over Normandy, darkness, clouds, anti aircraft gunnery causing both physical and mental damage, plus cascading delays from lost planes blundering into the path of other planes, made a mess of the mathematics. Hence falling back to relative positioning.

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TLDR, The planes behind, should be above the parachutists in front