As you can see in the above photo, the Cyrus McCully grant of 360 acres (A) can be found in B. O, p. 91 A. This means Book O p. 91 grant A. It turned out there were three grants listed together and the McCulley grant was the first grant listed and also the first drawing labelled A. I started by going to the Grant Register page on Family Search and discovered that Book O would be in film #008632288 which covered Books N-Q. Then I clicked on the icon for that film and even though it had a key on it signifying it was locked, it opened anyway. The film opened in thumbnail view and I knew the large Numeral 1 section would be Book N, so I scrolled down to Numeral 2 and clicked the next image which showed the back of the spine for book O. So I knew I was in the right section. Then I just used the image field at the top to jump around until I located the McCully grant on page 91 (Image 302). Then I used the arrow keys to arrow through two pages of the grant to the maps.
Sometimes a grant will only have a 4-digit number instead of a book and page number. In that case, especially if the grant was issued after 1859, scroll down the Family Search Grant page and look at the grant number ranges for books 17 and higher. These grants are all in numerical order.
If neither of those quick methods work, then you can always start from ground zero using one or all three of the indices given at the top of the Family Search page. I've created instructions for using the indices on the Land Grant Registers Online page of this website.