Why does Pip "embroider" his visit to Satis House? (chapter IX)
When people use the verb "embroider" to mean something other than its most common action (to sew a design) it is often meant to leave a mark.
In this case, the mark he leaves on the experience of going to Satis House is indeed a story (a lie) of grand proportions about what when on there.
It involved seeing Miss Havisham in a black velvet coach, eating cake on gold plates, great dogs and playing with flags.
I think he tells this story because he has been controlled and overpowered his whole life by Mrs. Joe. Never being allowed to have an opinion or thought of his own, not to mention, never a task of his own, he saw his first opportunity to have control and he took it. Getting away with any sin demonstrates power, not that this is a good thing. This is Pip's first experience to know better or know more or exert his expertise over Mrs. Joe, something he might have been longing for for a long time.
When Pip comes back from his first visit to Miss Havisham's house, Mrs. Joe and Mr. Pumblechook want to know what happened. But the actual events that happened when Pip went to Satis House were so weird that he is afraid they will not believe him.
So he tries not to tell them anything. But that makes his sister really angry so she starts to push him around physically. That is why he starts to make things up. He starts out with one lie and finds out that Mr. Pumblechook does not know about Miss Havisham's home. That leaves him free to tell whatever lie he likes.