How can I make quicksand ?

Do a block without collision and do a procedure when entity collides with block and you put the block "set entity for cobweb in one tick" in the entity blocks.

ı dont understand , can you explain please ?

It's as simple as it sounds. In the procedures on the left is "entity procedures". In there you will find the "set entity for cobweb in one tick" piece. add it under the main piece and save. Add the new procedure to the "when entity collides with block" In the blocks procedure page and save.

What I want to know, is, how do you make it so it suffocates you once your head is under the surface???

that wont work with spiders

Just want to create a block that you "sink" in and can't escape but don't receive damage unless your head goes under.