I was at a bar some time ago and I ordered a random shot. A B52 I believe. And it looked perfectly layered, no gradients between any of the three liquors. So how do the bartenders do it?
How can I layer liquors on top of each other?
The key to creating perfectly layered drinks is to pay attention to how heavy each ingredient is compared to the other ingredients. The weight of each liquid is measured by its specific gravity.
An alcohol density chart can be viewed here.
There are a few simple rules to follow:
You want to use a narrow glass to maximize the thickness of each layer without using a ton of booze. There actually is a specific pousse-café glass, but only your crazy aunt has those, and do you really want to go over there and borrow one? No. Just use a champagne flute or a skinny shot glass.
The order in which you pour is the most important aspect. You want to start with the liquid that has the greatest specific gravity, and decrease as you ascend. That way they liquids won't try to pass through each other, which will mostly likely just end in blending. Terrible stupid blending! Start with the sweetest, least-alcoholic liquid, and go up from there until you have the least-sweet, most-alcoholic liquid at the top.
Pouring technique is critically important. This definitely takes a steady hand. If you pour too fast, the top layer will plunge down into the one under it, which will cause mixing. That ain't pretty, and prettiness is what this drink is all about (prettiness and science). The top of a bar spoon is pretty much the ideal tool, but you can pour over the back of a regular spoon, too. Just position the spoon right above the last layer, and pour as slowly as humanly possible. This is why bartenders hate you for ordering these. How to make a multi-layered drink
Here are a few Tips for layering drinks:
To keep the layering effect, the drink should not be stirred.
A chilled glass often works best. Also, if your drink does not include ice, it's best to chill the ingredients before pouring.
A speed pourer can be helpful for slowing down the pour as well. This is particularly true if your liquor bottle is full and heavy.
Some bartenders will put the tip of the spoon just under the first layer, which helps the liquid stay on top. Depending on your technique, this may help you as well.
Layering can also be done with a syringe that is food-safe. It's not as professional (or cool) looking, but it is easier. Be sure to choose one with a larger hole. - How to Create Layered Cocktails and Shots
Use the back of a spoon to help you create your layered drink.
For those who desire to look more professional in this domain, one could employ a Rainbow Cocktail Layering Tool
An ingenious piece of equipment, the Rainbow Cocktail Layering Tool allows anyone to start making their own layered cocktails. Using a clever float that sits on your drink as you make it, you can easily pour one drink on top of another without the layers mixing!
Above all, please pour your liquors and liqueurs very carefully and slow.
It does depend on the liquor you're using since they have specific gravity's. You can pour them carefully over a spoon to minimize disturbance, or (what I always did) pour them slowly down the side of the glass, one by one in order (highest gravity on bottom).
This page spells it out pretty well: Specific Gravity Chart for Layering Drinks and Shots.