(a) One afternoon, Mei was waiting for her mother to get home. She was very hungry so she made some sandwiches. [full stop, not comma] First, ["Firstly" is not actually wrong, but it seems a bit stiff here] she open [wrong verb form; you should be able to fix this] a tin of tuna.
(b) After that, she sliced a cucumber and an onion. Then, she put all the ingredient
It was a bright and sunny morning. Susan was waiting for her mother coming back home from the market. Suddenly, she felt very hungry, so she decided to make tuna sandwiches.
First, she sliced the cucumber and onion with a sharp knife. Then, she put the ingredients and tuna in /into a bowl. After that, she mixed them well.
Here are a couple of comments to help you revise these sentences. Then, we can comment further.
Can I say,
It was a bright and sunny morning. Susan was waiting for her mother coming back home from the market. Suddenly, she felt very hungry, so she decided to make tuna sandwiches.
First, she sliced the cucumber and onion with a sharp knife. Then,
First, she sliced the cucumber and onion with a sharp knife. Then, she put the ingredients and tuna in /into a bowl. After that, she mixed them well. The phrase 'the ingredients' includes the tuna. You could say 'these ingredients', to refer to the cucumber and onion that you have already metioned,