To get to El Hierro, the fastest and most comfortable way is by plane. You have to fly to Tenerife, if possible to the north airport, Los Rodeos.
Unce you get to Los Rodeos airport, there is a flight from Binter Canarias (the only company that operates with the island) that links Tenerife to El Hierro.
Depending on the season there are usually between three to five daily flights from Tenerife to El Hierro.
Our advise is that you book the flights separately using some flight comparator like Kayak or Rumbo, from Tenerife Norte as origin.
Once you have the schedule and the flight for Tenerife North airport, take a look at the page of Binter Canarias to book the flight to El Hierro. Don't search for flight with origin El Hierro as you will get unreal prices and schedules.
You can always contact us with no obligation and we will try our best to help you. In that case, we will need specific dates, and departure airport. Sometimes according to timetables and departure/arrival airport (Madrid and Barcelona are discarded) is possible that you have to stay one nigth in Tenerife.
In that case we can give you some advice on accomodation at a very good price.
Another way to get to El Hierro is by ferry. There is one leaving every day (excecpt Saturdays) from Los Cristianos's Port, in the south of Tenerife.The journey lasts between 2 and 3 hours. It isn't much cheaper than the plane, but it's much more uncomfortable. Although, is one more option to keep in mind. The only shipping company that operates is Naviera Armas.