Get a big enough bag. If you can get it to float, just open the mouth of the bag wide, with the cut-edge folded outside so it won't contact the anemone, and nudge it gently into the opening. Give it plenty of water and transport it quickly: don't keep an anemone sitting in a bag any longer than necessary to drip-acclimate it: they can let loose some pretty nasty chemicals and haven't a brain to let them understand it's all the water they've got...