How do I make Minecraft more difficult?

You could make all blocks have gravity.

That would make building much more difficult.

Have a look at Terra Firma Craft mod. It's has some nice realistic mechanics, like rotting food, temperature, and harder crafting.

I would suggest increasing the health of mobs (I don't know if it's possible though to change the health of vanilla mobs in mcreator though), and making more factors for your survival (like your hunger and health bar).You could also make the player's inventories smaller, and make tools break faster.

you can make items crafting harder

or add water so you need to drink

Adding Scurvy, Altitude Sickness, Trench foot, and other conditions that would impede player progress.