Chrollo would slit the throat of the sacrifice. He didn't know who they were, nor did he care. But their sacrifice would be essential for this to work, so he's grateful for their (unwilling) sacrifice.

As blood oozes out of their throat, he'd put an empty, golden goblet up to their throat, collecting enough of their blood to mostly fill it. He'd let out a soft sigh as he stepped away from their body, walking over to your own lifeless body as you lay beside them on the pentagram drawn from fresh blood.

He puts the goblet up to your mouth, gently tilting it as he allows the blood to pour into your mouth. It was gross, but if it worked, it would revive you.

After he emptied the blood filled goblet, he stepped back, dropping the now empty goblet and closing his eyes as he waits to see if it worked.

You weren't in the Phantom Troupe, nor were you important to any of his plans. So why does he care at much about you? Well, you're his younger sibling. You were just as important, if not more important to him, than the members of the phantom troupe were, and they're almost irreplaceable to him.