Name the limit
Choose one situation. Think about the behaviour you want to respond to, rather than trying to change everything about the relationship. You can introduce a limit after knowing someone for years.
Keep the wording concrete
Say what you can offer or what you will step away from. A boundary does not require the other person to agree with every feeling behind it.
Notice the response
Respectful people may need clarification. Repeated pressure, threats or fear are different. If setting a boundary could put you in danger, seek confidential local support and prioritise your safety.