This is a quick and dirty poor man's guide to making a black and tan using Guinness and Bass Pale Ale. I nkow it isn't a beer review, but it's always handy to know. There are of course official ways to do it, that include this fine pouring spoon from Guinness themselves. While I'm sure it works brilliantly, a normal household spoon or equivilent will do just as well. So basically you just gather everything together shown below:
First open the Bass (or whatever...you can use just about anything including Harp). Simply open the bottle and fill your beer glass to the 1/2 point (generally this uses 3/4 of the bottle....good excuse to polish it off).

Now this is where the only trick is at. All you do is hold the spoon upside down and pour over the curved back of it. If you click on the picture below you'll actually see an animted gif of what this looks like:

As the gif shows make sure you use caution as the pouring of Guinness attracts natural preditors :)
Just remember to take your time when pouring, don't go to fast, and you'll end up having your self one impressive looking drink.