Just like tossing a rope to a person floating downstream, you need to time your entry so the two of you arrive at the meeting point at the exactly the same time.  The ideal position is within an arms length - just like tossing a throw rope.  Favor a tad downstream of the victim rather than upstream since you can backstroke to slow your progress and let them come to you.  Avoid arriving too far upstream as it can be difficult to chase the victim downstream.  This is a decent exercise to practice to get the timing right. The belay person needs to give you plenty of slack to maneuver.  Set aside a good 20' of line prior to the jump and be ready to quickly feed more.