How do make a cello electric?

On an existing cello, adding a pickup would be the simplest way.

There are several pickups available for cellos on the market. Doing a google search for 'pickup for cello' gets several results. It is saved as a bit.ly link, but those don't work in Wikianswers.

The URL for the google search is posted in the Related Links, but if you have problems with the link for the google search, then just go directly and do the search. I'm directing you to the search, because there are numerous available, and it's not the place here to recommend one over another.