Pinch the soft part at the front of their nose (or get them to) and ask them to lean forward and breathe through their mouth. Pinching the nose helps the blood to clot.
Don’t ask them to lean back, even though it may seem like the right thing to do. The blood could get into the windpipe and block their airway, or it could go into their stomach and may cause the person to vomit.
If bleeding persists, call 999 or get someone else to do it.