How can I make patterns like these?

Would you just create custom shape and make that as a pattern or how?

http://www.kcparker.com/images/site2/shared/body_bg.jpg

http://www.hartstrings.com/images/site1/shared/bodyBackgrounds/hartstringsBaby.gif

you can make patterns easily by either making your own custom shapes or importing something someone else made.

if you need it to blend seamlessly you need to observe the symmetry of it and be sure the edges fit from one "piece" to the other. Otherwise just select hatever you want made into a pattern and go to "Edit" -> "Define Pattern"
You can then fill another selection of your choosing with your pattern