Can't seem to get it to link but basically if you put Felixstowe ferry into Google maps as your departure point and Liverpool Pier Head as your destination that will give you a basic idea. What I would do rather than go the whole route by motorway is once you reach Birmingham then head for the nearby A41 and just stay on it - it will take you all the way to the Mersey Tunnel entrance. Go through the tunnel and on the other side you'll come to a roundabout. Take the first exit into Dale St, which goes slightly uphill. Do not turn off that road - it will start to go downhill again & you'll have to go through a few sets of traffic lights. At the bottom of the hill there are more traffic lights - be in the right hand lane. Across the road to you right you'll see the entrance to the ferry terminal but you can only go left at these lights - get in the right hand lane (outside lane) and after about 200 yards you'll see a gap in the centre line where you can head back the way you just came. Be in the left (inside) lane and the ferry is (from memory) 2nd left. Check-in will be on your left.

Sorry for the long winded directions but I couldn't seem to get a screen grab - too old & dumb