Make a brine by adding Kosher flake salt to water. put a potato in the brine. It's salty enough when the potato floats. Put the live eels in the brine and let them stay there for a few hours. They will eventually die, get salted on the inside, as well as the outside, and the brine will de-slime them. After a couple of hours, remove th eels from the brine and put in air tight container, layer of salt, layer of eels, salt, eels. Brine is re-useable
You can get salt at a feed store, that works well and is a lot cheaper.
Some fish bait and/or tackle shops also carry it.