I went to buy cheese, breads and one (kitchen ?).

It is unclear what you meant by kitchen.

I came back home,. The first thing I did iwas baked a cake with appele, then I prepared the dough, prepared the filler and formed on a (triangle, ?), and then I put them in the oven.

I’m unsure what you meant by triangle. Perhaps you meant you prepared the filler and formed a triangle, or formed the filler in the shape of a triangle.

Finally I prepared chicken thighs and potato in the oven.

I went to buy cheese, breads and one kitchen.

everything is correct except "one kitchen". I do not know what you mean.

I came back home, and the first thing I did iwas baked a cake with appel, thenle. Then, I prepared the dough, prepared the fillering, and formed on a triangle, t. Then, I put ithem in the oven.

Finally, I prepared chicken thighs and potatoes in the oven.

You can say "a potato" if singular or "potatoes" if plural.

I went to buy cheese, breads and one kitchen____.

'Bread' and 'breads' are both plural. If you only bought one type of bread (e.g. croissant), then don't add 's'. However, if you bought, say, a croissant and a baguette, use 'breads' now that there are two types of bread.

Also, I don't you bought a 'kitchen'. Maybe you meant 'chicken'?

I came back home, first thing I did iwas baked a cake with appel, then prepared the dough, prepared the filler and formed on a triangle, then I put them in the oven.

Finally, I prepared chicken thighs and potato in the oven.