I consider Mitchell's "Will Index" to be my #1 resource material to researching NC families. It is a two volume set that sits out on the main counter at the NC State Archives. It can be searched online using the NC State Archives MARs search Engine. This is how I went about finding the last will and testament for Henry Williams:
Go to NC State Archives webpage
Click on the MARs Catalog link on the left side and a new window will open. It may take a couple of minutes to completely load. You will see the main search page. I have included a screen shot below:
Next I type in "Henry Williams" in the search box and I put a check mark beside the Mitchell Will Index listing. Then I click on the search button.
The database loads the Mitchell Will Index and reports back that there are 7 hits in the County Records Part 1 section and 9 hits in the County Records Part 2 section. This index is alphabetized by county name. Counties starting with a letter in the first half of the alphabet are in County Records Part 1 and those in the second half of the alphabet are in County Records Part 2. The actual published copy of the index is alphabetized by last name.
If I wanted to look for a will for Henry Williams in Caswell County, NC only, then I could type in "Henry Williams Caswell" into the search box and it would bring up 1 hit in County Records Part 1.
Click on the County Records Part 1 link and I get a display of 1 record for Henry Williams.
Click on the blue "I" icon in the first line
The record depicts information for this Henry Williams. The will was recorded 1786 in Caswell Co., NC. The call number and MARS Id number are not important.
Once you find the will date, person's name and county, you can go to the Archives and look through the will boxes. They are organized by county first, then alphabetized by last name. Before you can look through the records you have to fill out a call slip. The call numbers start with a 3-digit prefix assigned to the county of interest. Caswell County's 3-digit prefix is 020. Then the next 3-digits are assigned to document type. For wills, that number is always 801. The last number of the call number is the box number that contains your ancestor's file. This last number is left blank on the call slip. You will put your surname on the call slip instead. They will bring you the box that contains the surname for the county of interest. For example, I might find the will of Henry Williams, 1786, Caswell County, using the following call number: 020.801.25. Box 25 containing the surname Williams.
The archivist will give you the whole box and you can only take out 1 file folder at a time. You must keep all documents inside the folder in order at all times. You can request copies be made by filling out a copy request form. Copies are $0.10 a page. Information on locating estate records at the archives can be found in my post "Locating Estate Records at the NC State Archives."