Hi OP. I am a breastfeeding peer supporter and we also cover expressing.
Firstly, can I just say what a wonderful thing you did for the parents, and that it's doubly wondeful that you are doing your best to provide breastmilk.
Generally many of the things we suggest for helping to express won't work for you because of your situation.
For expressing to work well, you need oxytocin, the 'feel good' hormone. This is usually triggered by a cuddle with baby, smell of baby etc. In your situation you need to find another way.
Try skin to skin with a partner if possible, ir if you have a pet, give your pet a cuddle (not necessarily naked!)
You can try watching a feel good/romantic type of film. You could also try looking at photos of your own child as a baby (if I understood correctly that you have one). Thing back to how small they were, how it felt to cuddle them, how they smelled.
You can also try a hot water bottle on your breasts, and some breast stimulation. Massage your breasts but particularly play with your nipples.
At this stage if you are trying to harvest colostrum you may be best hand expressing as you are likely to get a very small amount until your milk comes in, and it may get lost in the pump.
Good luck!