Jenna, my little Macchiato ................ BM knows exactly what I mean. Yes, you really do need a piece of hose or similar. Garden hose connector, or one of those rubber shower attachments that push onto the spout.
As he says, the best way is to push a hose onto both sink or basin taps......... then slowly turn the cold on until it forces the airlock in the hot back up to the roof tank.
A few times, I've gone under the sink and cross connected the H & C plumbing temporarily to do it................ but I wouldn't advise it. You could get into a real mess.
Try and borrow a rugby player and a short length of hose :o)