Drip acclimation is a process by which you 'drip' water from the main tank slowly into a container with the critter.
Here's what many do:
Float the bag with critter in the tank with the lights off for 15 minutes.
While that's going in, place a clean bucket on the floor in front of the tank.
Grab a piece of airline long enough to reach from inside the tank down to the bucket, and tie a knot in it.
Clamp the top end of the airline to the top rim of the tank, with the end down in the water, and the lower end of the line into the bucket.
When the 15 minutes are up, dump the contents of the bag into the bucket, making sure the bucket is sized so that the critter is completely submerged.
Suck on the lower end of the air line to get a siphon going, and tighten or loosen the knot to regulate the drip so that you get a steady but slow drip.
Once the volume in the bucket doubles, take 1/2 the water and throw it away.
Repeat until you monitor the salinity, and ph, and they match the tank.
Then transfer the critter to the tank. For sponges, anemones, urchins, stars, and some fish it's important to not expose them to air, so use a small container to scoop them up.