How to make a new model to a existing assembly ?

Hello (Simple question)

How to make a new model to a existing assembly ?  (Inside the assembly)

• Assembly->Create New, but what from there ? I'm not sure ?
• Now the new model is in "work part" mode... I pressed Create Sketch, but how can I select a plane withing the existing assembly when everything is greyed out but only the newly created model ?

• How to use the features of the existing assembly to create the sketches of a new model ?

Now:

1. I make the model outside the assembly....(Without knowing how to use the existing assembly as a reference to create the model. So no reference from it. I just remember different measures from my notes and from my memory...)
2. Then Add the model to the assembly...
3. Then Move it to the right place.