Once you get to Algeciras, it should be 100% obvious for anyone to arrive to the harbor and hop on the next avaliable ferry. There is no complication whatsowever. I need to point out though that there are peak days when all the muslims from Europe go down to visit their families (usually on days 1-5 and 25-30). Expect a few hours on the line. Besides that, is very simple, effective and cheap.