You don't. Undersized runts should never be bred.
95% of purebreds should be fixed (and ALL healthy mixes). Only purebred dogs who are two years old and up, have been titled, are ambassadors of their breed, and have passed through their genetic health screening with flying colours, should ever be bred. A new breeder should never attempt to breed if they do not have a mentor. Also, if you are serious about breeding; help out with rescue. A responsible breeder's knowledge of whelping comes in handy if there is ever a puppy mill bust, etc.
Breeding information, and the realties of careless breeding:
http://www.brightlion.com/InHope/InHope_en.aspx
http://www.youtube.com/watch?v=emGZBLVJmGI
http://ca.youtube.com/watch?v=hpAXRx8FRRE&watch_response
http://ca.youtube.com/watch?v=1bpDE6Vx_QU&feature=related
http://www.woodhavenlabs.com/breeding.html
http://www.geocities.com/bluegracepwd/virtualbreeding.html
http://www.learntobreed.com/
http://www.dogsindanger.com/
http://www.wonderpuppy.net/1breeding.php
http://www.dogplay.com/Breeding/ethics.html
http://www.petfinder.org/ has 286,376 pets in need of homes, new ones added every day, not all of them are in safe no-kill shelters.
Be a part of the solution . . . Not the problem.
http://www.spayusa.org
**EDIT** Thanks for wanting to breed RESPONSIBLY!