I understood everything but not the sticker in which a phrase "come by after- Natalie"..Like who would have put that sticker in his jacket? I don't seem to get the answer till now. Jimmy seems to know Lenny before..
The note was written by Natalie, and given by her (off-screen) to her boyfriend, Jimmy.
The movie implies that by "after" the note meant "after you make your drug deal." So Jimmy was supposed to go see Natalie after the drug deal. But during the drug deal, Jimmy was killed by Leonard.
After killing Jimmy, Leonard put on Jimmy's clothes and assumed ownership of the other stuff he had on him, including the note. And then Leonard read the note and decided to go to the address on it.