1. You don't have to unplug the electric cord to the pressure cooker. You can see the light of preserving temperature on the cooker. If you unplug the electric cord, the cooked rice in the cooker will be spoiled with ease. (Is this passage correct? Would you correct the wrong expressions?)

2. Let's prepare for going out.
3. Let's prepare going out.
4. Let's prepare to go out.
(From 2 to 4: Which expession is right?)

5. By making a wish, you throw a stone on the surface of the water. However, in our country, we don't have such a custom. (Is this passage correct?)

1. You don't have to unplug the electric cord to the pressure cooker. You can see the light of preserving temperature on the cooker. If you unplug the electric cord, the cooked rice in the cooker will be spoiled with ease. (Is this passage correct? Would you correct the wrong expressions?)

What is "preserving temperature"? In the second sentence, the repetition of cooked/cooker is awkward; delete "cooked" and keep "cooker." I'm not sure what you mean by "with ease."

2. Let's prepare for going out.
3. Let's prepare going out.
4. Let's prepare to go out.
(From 2 to 4: Which expession is right?)
#4 is correct; the others aren't.

5. To make a wish, you throw a stone into the water. However, in our country, we don't have such a custom. (Is this passage correct?)

1. The passage you provided is grammatically correct, but the expression "the cooked rice in the cooker will be spoiled with ease" can be improved. You can say "the cooked rice in the cooker will spoil easily" to make it sound more natural.

2. The correct expression is "Let's prepare for going out." This conveys the idea that you are getting ready in advance to go out.

3. The expression "Let's prepare going out." is not grammatically correct. It is missing a preposition.

4. The correct expression is "Let's prepare to go out." This indicates that you are getting ready for the action of going out.

5. The passage is grammatically correct. However, if you want to avoid repetition, you could rephrase the sentence as follows: "In our country, we don't have such a custom of throwing a stone on the surface of water to make a wish."