Taking the ferry between Algeciras and Tangier Med is quite simple. From the Algeciras, Spain side;
Drive down to the port, and at the roundabout take the parking lane. You will get 30 minutes of free parking here. The tickets can be bought at the buildings ahead of you next to the parking lot. You have 2 choices. Go the building on the right, and buy from one of many brokers who will be able to sell you on any boat. Or, go the building on the left, with the café, and buy direct from the shipping companies. Buying direct will be cheaper, and have better refund options if you need it. But, you have to know which boat you want. This is normally simple... you want the next departure.
Once you have your ticket, go back to your car. leaving the parking lot; turn into the roundabout again, and take the other 2nd exit towards the checkpoints. First, pass thru ticket control. They will put a placard on your windshield showing which company you are going with. Then, follow the lane and you will be directed to the proper waiting lines for that company. That is all. Then, just wait for your ferry. This was quite quick (less than 30 minutes) when I was there in September. It will be longer in the summer months.
Going to Tangier, you will have Moroccan passport control on the boat. Go to the back of the boat, on the upper level, and there will be a table in the lounge. In the busy season, do this early as the line can be long.
Leaving the boat in Tangier. If you do not have a Moroccan car, you need to pull forward to the passport control (past the scanner) and get a little white piece of paper from the agent. Then, drive back down towards the scanner and wait your turn.
Then, back to the same booths as before, and get the final driving permit for your car. This process can take a while, and will be done in Arabic only. I dint find any agents that spoke Spanish. After you get your permit, you follow the road towards a row of shops. Here, you can get cash, snacks, bathrooms, and insurance during normal business hours.
Going from Tangier Med to Algeciras;
This process will be much longer. In September, a little bit of a slow season, it took me 90 minutes. So, arrive early. If you don't have a ticket, when you arrive at the port; there are shops at the first parking area. But, if you were like me, I bought an open return ticket, so I can by-pass this.
First stop after shops is ticket check. After ticket check pull forward to passport control.
Then, just pull forward to the next stop - customs. Here you will need to show that little white card you got when you arrived. They may also search your vehicle.
After passing thru customs, you will pull farther forward and have to get scanned again. this is where it really bogged down for me. The wait was about 40 minutes for the scanner.
Once finally past the scanner, go tp the roundabout, turn towards the Quays, and go to your designated parking lanes to wait for your ferry. The parking area will be on your ticket.
On the ferry this direction, there is no passport control on the boat. It will happen after you arrive in Spain. Getting off the boat, follow the exit signs, go thru passport control (I did not get a stamp), then onto customs. When, I arrived at customs, I was simply waived through...no papers, no checks, no nothing. And, onto Spain.
Something to keep in mind, my experience was that neither ferry left anywhere on time. They were both about an hour late. Also, the 1 hour trip actually took about 1 hour 45 minutes.