Have never tried this but have thought about it since there is a girl I know who I would also like to obtain a pair of her dirty panties.
Only thing I can think of is you need to be invited to her house, possibly a party with lots of people.
Now depending where her dirty clothes basket is, will make this tricky. Hopefully it’s in the bathroom so you can just grab a pair when you use the bathroom.
Unfortunately it is probably in her room. In that case you will need to excuse yourself from the party to “make a personal phone call”. It’s up to you if you want to ask to use her quiet bedroom for this, or if it’s better just to sneak in there. Important!!! If you choose to sneak into her room during the party be sure to have your phone on your ear so if you are caught in there, they will see you are on the phone; you can even whisper something like “had to take this call..”
Good luck.