we would say you were cooking from scratch, "scratch."   我们就会说你是从头烹饪，“从头开始”。

To cook something from scratch means that you make the entire thing yourself.  从头开始烹饪食物意味着你一个人做整件事情。

Well, Lucy is not making cookies from scratch; she's using ready-made cookies, which she takes out of her "fridge that just need to be baked,  露西不是从头开始做饼干的，她用的是从现成的，从“冰箱”里拿出来需要烘烤的饼干。

that is they just need to be put into the oven and they will cook.  也就是说他们仅仅需要放进烤箱里烘烤。 

She puts the cookies on a cookie sheet.   她把饼干放在烤盘上。

A cookie sheet,“sheet” is a long, flat pan that you can use to put things on when you put them in the oven.  烤盘，“烤盘”，是一个长而扁的器皿，当你把它放进烤箱时，可以用它来放置东西。

We use that term, cookie sheet, even when we are not using it to bake cookies.  即使我们不使用它来烤饼干，我们还是使用烤盘，这个术语。

You can put chicken on a cookie sheet and put it in your oven; we still call it your cookie sheet. 你可以把鸡肉放在烤盘里，然后放进烤箱；我们仍然称它为你的烤盘。

Lucy puts some aluminum foil on her cookie sheet.  露西把一些铝箔纸放在她烤盘的上面。

Aluminum, "aluminum," is a kind of metal.  铝，“铝”，是一种金属。

Foil, "foil," is a metal sheet - a very thin sheet of aluminum. 锡纸，“锡纸”，一种金属薄板——非常薄的铝片。