How to achieve reentry with bigger crafts?

After getting the basics of the game down, I decided to grab some tourist missions for the money. I decided to take 2 missions (6 tourists) in a single launch.

I have tried multiple designs but nothing worked, the ship would flip around and destroy unless I constantly steered it

And say I wanted to go bigger, maybe re-enter with my entire Minmus lander, or get those Science Jr.s back how could I survive reentry?

I landed this time by spinning my craft wildly to stop the heating, but some just don't turn as well and get stuck pointing prograde. Is this strategy possible to replicate consistently or should I plan a well structured reentry module, and if so, how? Also does the trajectory drastically affect reentry?

Craft design

Yes, you should plan for reentry if you'd like to land something big intact. Generally craft reenter like darts - heavy end forward (so center of lift will be behind center of mass).

If you have fins/winglets on the bottom of your lander, it will be almost impossible to reenter any other way than nose-first. So drop them with your boosters.

Other than that, if you'd like to reenter engines-first, then place all the heavy parts there (which will be mostly the engines, but crew compartments are also heavier than empty tanks or science labs). If you have parts which stick out (and produce drag), like the goo containers, try to put them closer to the nose.

Also, before reentry, if you have any fuel left over, transfer it to the tanks which are closest to the end you want to go first (so they are the heavier ones). To transfer fuel between tanks, right click on both of them while holding alt.

Trajectory

Trajectory is also very important for heating. Generally you'd want to spend quite a bit of time flying horizontally in the atmosphere to slow down. Altitudes below ~30km will very likely cook you at orbital speeds. Altitudes above ~40km might not slow you down enough for landing. So for reentry from the Mun or Duna you'd generally set your periapsis to about 35km, which should generally result in you slowing down to somewhere around 1km/s at those altitudes, and at that speed further descent will not heat you up significantly.

Note these numbers are for Kerbin, other planets have different atmospheres.

However, if you're coming back from, let's say, Jool, your speeds can be upwards of 4km/s, and you will cook even at 35km altitude. In those cases you would aim a bit higher, perhaps 45km, which wouldn't slow you down enough for a landing, but enough to not escape Kerbin again. And then slow down over multiple passes through the atmosphere.

Gedas' answer mostly covers it. But I wanted to add a few other notes on surviving entry:

1. Vessel width: A wider vessel, say 2.5m will slow faster than a narrow vessel because of air resistance.

2. Vessel length: This isn't really about length, but weight. A heavy vessel will slow slower...

3. Vessel angle: Scott Manley, patron saint of rocket science (praise be upon him) did a fantastic video on how vessel angle can generate aerodynamic lift, increasing your time in atmosphere, and hence decreasing your heating.

4. Wings: Following on from that, I have added winglets, in reverse, on capsules to control heating. It's not a good idea if you don't have fairings to start with (covering the winglets on launch) and it won't work if you plan to get orbital again, but it can help.

5. Service bays: Now we're really getting advanced. If you hide your winglets inside a service bay, closing the bay will 100% eliminate the lift they generate. That can be helpful if you want to be able to launch without fairings and have control on the way back down. Or if you want to repeated launch without your winglets flipping you backwards, but land (with bay doors opens) with some control.

6. Drogue chutes: If you manage to survive entry heating but are still spearing into the ground at 500 m/s, try adding a drogue chute.

7. Spin: The best star-pilot in the galaxy once said, "I'll try spinning, that's a good trick". He was right. Sometimes, spinning can help decrease heat stress.

8. Piloting is key: 99.9% of the time piloting a vessel will increase the chances of survival. If you're having a problem with heating, you will certainly die if you're not actively flying.

9. Heat shielding: KSP has kindly included heat shields. Use them. And make sure they're pointing towards the heat, they don't work if you put them in the wrong direction.

10. Decrease weight: A heavy vessel slows slower. Trying dropping engines, fuel, fuel tanks or any other dead weight if you can. Land as little weight as possible.