[Qgis-user] seeking help on QGIS symbology for clorepleth maps

[Kuan Song]

Dear community,
I hope to seek help with QGIS functionality on symbology. The goal is to create simple maps with variable-sized symbols for a polygon map. For example: a map of counties, with bubbles on top indicating the population size in each county. This can be very easily with ArcMap, but is not very easy with QGIS.
Yes I know it should be under the 'styles' tab, and it has something to do with adding a new symbol. But the part that's baffling with the user interface is: how do I link a symbol size to a data column in the data shapefile?
Yes I know there is a QGIS manual. However, the current manual is very outdated. Many parts of it are simply copies of the manual from older versions, and are utterly useless. For example: http://docs.qgis.org/2.8/en/docs/training_manual/basic_map/symbology.html shows a different UI layout to the current software 2.14. 

This is a trivial and elementary problem. I feel ashamed asking this question. I'm sure many people have encountered it before. Could someone point me to a page where I can find a walk-through with actual screenshots of the current QGIS version? That'll be very helpful.
Thank you.
And again, to the QGIS developers: Please do not let poor documentation ruin a fine software product......

YoursKuan
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