I am no expert but, I have a few tips that may help you:
Remember as JohnMarshall said the stronger the filter the closer you will need to get. Also the focal length of the lens will affect the distance from the subject. The lower the focal length the closer you will need to be. So if you use a 50 1.8 (that is my favorite) you will be reaaaaaaaaaaaaaaalllllllllllllll close before you get focused. Don't get discouraged, it takes practice. When I started LordV gave me some great advice on focusing. Move in and the image will come into focus; go a little closer to blur it; then slowly move back out. When the image comes in focus shoot! Experimenting with aperture will pay off as well. The higher the F number the more depth of field you will get. Be careful when getting to f16 or higher as you begin to loose image quality. I had to make a homemade "flash extender" as I do not have a flash unit yet. Do a search and you will find several good examples with instructions on this site. Most of all keep clicking, clicking and clicking.....it will come to you.
I found it much easier to use extension tubes! Better IQ as well.
Alan