Ray you need to give them two things
1. your moddified pattern, this will have the hole filled in solid and the core print sticking out of each half. When they mould the sand around this the filled in bit obviously forms a void and the print leaves a place to locate the core.
2. You also need to supply a core box this usually a split box into which they will place sand to form the core, needs to be split so they can get the core out. The sand in the core is different and has additives which allow it to be bonded into one hard piece.
Process is they make the core in the corebox and then remove it. Next they ram up one side of your pattern, turn the box over and place the second half of the pattern ontop and ram sand around that. They will then take the two halves apart, remove your pattern halves then place the sand core into place and put the two boxs back together ready to poor. The sand core is held by the print (a sort of spigot) which keeps it away from the edges of the sand so iron can flow down the gap and form the sides of the part