First off, take some pillows or a nursing pillow and put it on a relativly clean area of the floor. You want to make sure that any light coming through a window is coming from behind where you plan to stand to take the pictures, not from in front of. Also, if it is a bright enough day try to stick with the natural light of the sun and not use lamps or overhead lights, this will give your pictures truer colors that a light bulb allows.
Next, find a plain sheet. It doesn't have to be white, but white is what I recommend since that way it won't give the skin a false color tint the way a colored sheet might. Take that sheet and drape it over your pillow(s), this gives you a professional looking backdrop for your pictures.
Next take tons and tons of pictures. Make sure you have lots of fun while doing it. Don't let this become a stressful activity for you or baby because then he/she will probably become fussy. Don't worry about trying to get just the perfect shot, because eventually, if you take enough shots you're bound to end up with at least a couple that you love. The great thing about a digital camera is you can just delete all the really blurry ones and the ones you just don't like.
Lastly, upload them to your computer and enjoy!
(If you want to really edit your pictures there are plenty of free programs you can download, and many computers and cameras come with basic photo editing software.)
*When doing this with children or toddlers, I suggest taking your sheet to the wall and using another for the floor to create the professional looking space.