Why do this, since even stock plans cost you at least a couple of hundred dollars each? Because the right stock plans can save you tons of time and money if you have a talented architect. Most plans are at least partly modular these days, and often you can even order prefabricated sections pegged to a specific house plan.
Remember that plans are not necessarily "as is." Builders can use a reverse set of plans (sometimes called a mirror image) to better site a house. It's also possible to hire an architect or modification service to personalize a plan. Ask if reproducible prints or electronic CADD files are available for the selected plan. Either will make the alteration process quicker and easier.
There are a few aspects that you must finalize in your mind, before looking at the different online designs. You need to visualize the structure of the house with respect to whether there should be a basement, how many storeys you would like to have, the number of garages that you want, whether you wish to have any outside water pond, the size of the house, the sizes of the different bedrooms, kitchen, living area, etc., and the amenities that you want like bathroom tubs, sinks, and fireplace.
Drawing a good set of custom home plans involves integrating many different concepts and ideas into a solid and usable working plan. Many designers are able to come up with a unique or interesting preliminary design, but fail to deliver when its time to detail the house structure. They draw a pretty picture but have little or no understanding of how to translate that nice design into a working plan which the builder can understand.