Depends on how long you have and what method you use. the recomended way is with a drip line and air valve. Float the bag(s) for 20 mins, then empty them into a bucket. Start a syphen through the airline into the bucket and use the valve to adjust the flow to 2 drops per second. Leave it to stew for 2 hours them move them to the tank. The starfish must remain submerged at all times, so use a cup for the transfer. The urchin must not be touched. You will need to have the needles removed at the doctors if you do touch it...
All the best
Rabbut