Understand spices, their flavours, compliments and quantities

1) Write a list of 10 spices you'd like to understand better.

2) Buy a sample of each spice.

3) Learn to recognize the spices by their scent.
Sprinkle a little of the spice on your clean hand. Rub the spice with a finger or thumb to release the aroma. Smell it carefully as not to draw any spice into your nose, just the smell. Try to describe the smell by comparing it to other smells you have experienced in your life. Write down your thoughts and experiences.

4) Taste each spice and learn to recognize the taste.
Put a little of the spice on your tongue and let the taste fill your whole mouth (as different parts of your tongue/mouth react to different parts of the flavor, and your nose is involved in the experience of taste as well). Again, try to describe the taste by comparing it with other things you have tasted. Again, write down your notions.

5) Cook with the spices. 
Make at least 4 dishes with each spice and notice how the spice acts in the food.
Vary the amount of spices in a favorite dish and notice how the character of a dish changes.
Try different combinations of spices, and notice how the dish tastes and smells.
Make an unspiced version of the food and add spices, one at a time, let the food cook with the spice some 10 minutes and taste it again. Compare the taste with the spice to the taste without the spice.
Write down your experience.

Also, try with different parts and different preparing methods.
For example, dried garlic tastes different from fresh garlic. Garlic leaves taste different. Roasted garlic tastes different from boiled. Coriander seeds taste different from coriander leaves (cilantro).
How does a spice/herb work in a marinade?
Try roasting the spices before grinding them. Some spices should not be roasted, like paprika. Why? What's the difference in flavor? 
Some spices, like curry powder, are better if roasted or fried first, before other ingredients are added. (And "curry powder" is really not a spice but a spice mix, and you should be able to make your own spice mixes. Find out the ingredients of your curry powder and what these different compounds add to the finished product, and what would happen if you replace some of them with something else, or if you change the quantities...)
Taste the difference of "top spicing" and "bottom spicing". ("top spicing" being adding spices in cooked food, "bottom spicing" being starting with frying/roasting the spices and building the food on top of it.)
Some spices change character as they get heated up. Taste the food straight after adding the spice, 10 minutes later, half an hour later...
How does a spice/herb work as sprinkled on top of the finished dish? (It's usually fresh herbs; salt and freshly ground black pepper, paprika or cinnamon that's sprinkled on top of dishes, but why these and what else could you do?)

Also, learn to know the difference the amount makes. 1 teaspoon of cayenne pepper might make the food inedible, 1/4 tsp quite enjoyable... With some spices and herbs a little takes you a long way, others one needs to be generous with.

Also, the age of the spice. Some spices take storing well, and can be used to give flavor to food a long time after their expiration date. Some spices are barely useful to their expiration date. Generally it's good to store spices in as complete form as possible and grind them into powder only when you use them. Experiment with this. Some spices get a nasty bi-flavor
Freshly ground black pepper has some extra tones the ready ground pepper doesn't have. They disappear rather quickly, I think, so in everyday cooking it doesn't make much difference. Again, that is my personal opinion, and I am not a spice expert. Perhaps I will change my mind after having experimented with spices and learned "to understand the spices, their flavors, compliments and quantities".

This is a flavor wheel, created to help with wine tasting, but works somewhat with spices as well. Go through the different things described on the wheel, by smelling and tasting them.
Yes, I really want you to take wood chips and cut grass in your mouth and taste them, and chew them to get more flavor out of them. Do the same with tobacco, leather and earth and stones (don't chew those, though), but don't taste petrol. :-D

Now, to say cinnamon tastes cinnamon, pepper pepper, vanilla vanilla and mint mint is useless. Try to describe the flavors differently. You might want to describe mint with words like toothpaste, but most toothpastes are flavored with mint (or peppermint), so you are basically still saying mint tastes like mint.

The thing with this is as with every other form of tasting, wine, cheese, tea... the more you taste and describe what you are tasting and smelling, the easier it gets to describe and understand what you are tasting. So don't give up just because it feels difficult to describe anything. It gets easier as you go on. :-)

Another thing you could do, is to learn by heart the spice and herbs guides or primers, you know those lists of how to use which spice, or which spices to use in what food. Those lists are very cautious, giving only the most basic, common uses of the spices, but it's a good ground to start with.

After you are familiar with your chosen ten, choose another ten and get familiar with them.

If you want, you can learn the botanical names of these spices and herbs; the different medicinal uses of the spices; which parts of the plant are being used, and other such information that really doesn't have much to do with understanding spices in cooking. :-)

Here's more information about the subject:
How to use spices in cookery