The first step of the process is to determine if a bygdebook exists for the area that your ancestor lived in. In order to do this, you will need to know the name of the kommune (municipality) that your ancestor lived in. This is because bygdebooks are cataloged by kommune, not by parish.
The easiest way to search for a bygdebook is to use the resources I have listed on my Bygdebøker web page. Once you have the title, author's name, etc. of the bygdebook that you would like to borrow, bring the information to your local library and ask the librarian to do an online search of the holdings of other libraries. This type of search is called an OCLC Worldcat Firstsearch(r). (OCLC is an acronym for the Online Computer Library Center, Inc. This is a nonprofit library computer service that allows its members to search the holdings of several hundred other member libraries). Not all librarians are familiar with the term "OCLC Worldcat Firstsearch(r)", but if you explain that you want to do an online search of the holdings of other libraries they will usually know what you are talking about, or refer you to another librarian who is familiar with this type of search.
The OCLC Worldcat Firstsearch(r) will display a list of libraries that have the bygdebook that you want, and will also identify which of these libraries allow their bygdebooks to circulate by Inter Library Loan. (As I mentioned earlier, the University of North Dakota has a very large collection of bygdebooks, but it does not circulate these books by Inter Library Loan). Some of the libraries that do circulate bygdebooks by Inter Library Loan include the University of Minnesota-Twin Cities (Minneapolis), the University of Wisconsin-Madison, and Luther College in Decorah, Iowa.
You will have to fill out a request form and there is often a small fee for an Inter Library Loan. Then you sit back and wait for about two weeks until your local library calls you to let you know that the bygdebook you ordered has arrived!