We took that ferry about a year ago from Baku. It is essentially designed to move trains across the Caspian, not people or motos so it does not run on a fixed schedule. If the train was delayed in Turkmenistan or China the ferry sits and waits for it. You can't make a reservation ahead of time.
First you have to find the terminal. Go north along the waterfront from central Baku on the main drag, don't turn here..
This might be it (see the tracks?) but no....
It's actually this one, totally unmarked.
You'll see this after you turn. Still no indication there's a ferry anywhere.
To the right, down at the end of the road in the last pic, just past a gated arm across the road, is a little office where you arrange your passage. They are not too helpful, but will tell you to check back morning and afternoon daily to see when a ferry might be arriving/leaving. We were lucky and only had to wait a day and a half for a boat. Some people have reported waiting several days to sail.
You will need to put your bike in the Customs Impound lot (just a little parking area behind the customs gate) while waiting for your ferry to arrive. They inspect your papers, and do some other official hurry-up and wait stuff, then you're on foot/taxi in Baku until you sail.
We decided against the 5-7 day transit visa for Turkmenistan and got the regular tourist visa and a guide because there are so many problems with trying to arrange a tight schedule getting across the Caspian.
Then load your bike...
climb up the side...
Our cabin was ok but it was hot and stuffy. The toilet worked (mostly).
I ended up sleeping up here in the breeze on this old pedal power paddlewheel life raft. (They did have a few more modern lifeboats around, too.)
We had smooth sailing and were fortunate that when we arrived on the Turkmenistan side there was no other ship there. There's only room for one, so if it's occupied you'll sit and wait out in the water for a day or more waiting for a berth.
Then you'll start your adventure with the 12 stations of the Turkmenistan customs office.
Hope this helps a bit.................shu