OK, part three. First something I really should have covered sooner. Saving.
You can save the project using File - Save the way you would in most user software. QuantumGIS has its own file format for projects and they use the qgs file extension. It's important to note that when you create a layer, you are not making a copy of the data from the file, you are merely pointing QuantumGIS at it. If you were to edit the shapefile and load the project up, the changes would appear in it. Also, you can't delete or move the shapefiles without breaking the project. The symbols however are all stored in the project file so we have not so far changed the shapefiles in any way even though the way they look is now quite different. If you were to load one of the shapefiles in another project, or even load it again as a new layer in the same project, it would be given a new style.
Now, as of the end of Part 2, you should have had something looking sort of like this (Depending on how you chose to symbolize)
Now those layer names are a bit unpleasant. You can change them to something a bit nicer by right clicking and choosing Rename. These names will be used in the legend when we create one.
Now, open up the style properties for the cities layer. You should see a combo box in the top left that currently reads "Single Symbol" change that to "Categorized". It should now look like this
This will let us set up a style that chooses between different symbols based on the values of one of the columns in the attribute table. You can pick that column with the Column drop down. To start, pick ADM0CAP, which is 1 if the city is a national capital, and 0 otherwise. Under that are two controls to pick a symbol and a colour ramp. These will be used to give you a default set of symbols when we break the data into classes. You can change them if you wish.
Click the Classify button, and you will get a list of three values and symbols.
These were created by looking for all the unique values in the data. In this case, 0 and 1. An extra item was created for everything else in case a new value is added, or you alter the classes so they don't cover everything.
We don't really need all three so delete the class for 0 (not a capital) we'll let that be covered by the "everything else" class. Now, you can edit the remaining classes. Double click on the symbol to edit it the way you did the single symbol styles. Double click a value to change the value that a class covers, and double click the label to change the label for the class which will be displayed in the legend.
You can do the same thing with other other layers where they have an attribute with a restricted number of values. Rivers have ScaleRank for instance.
Some attributes have too many numeric values for each one to have its own class, the country layer has a GDP attribute for instance (GDP_MD_EST). Open the style properties for countries and change from Single Symbol to Graduated.
This is similar to Classified but you'll note the new Classes and Mode options. Classes controls how many classes you'll create, and Mode controls how it will try to break things into classes. If you want the details you should probably start by learning some Statistics but roughly speaking, Equal Intervals does what it says and breaks it into equal sized chunks of the maximum, Quantile tries to put equal numbers of features in each class, and Natural Breaks and Pretty Breaks look for natural clusters in the data.
Pick GDP_MD_EST, a suitable colour ramp, class count, and mode. Then click Classify and make any adjustments you want.
One final little thing for this section, you can group layers together. Right click on the layers list and Add Group. You can drag layers into groups, and drag and rename the groups themselves the same way as layers.
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