You can follow those practices to do that:
1. Run the program and click “Tools -> Settings -> Generating Index” and set the option “Ignore words containing less than” to 4 letters. This will eliminate all words having less than 4 letters in the book, like “the”, “run”, …
2. In Step2 select the second option “Index specific words in the book” and choose to Exclude those categories:
Adjectives – Adverbs – Basic English words – Conjunctions – Contractions – Interjections – Prepositions – Pronouns – Verbs
And please enable the option “Exclude words plurals”, so if “ball” is excluded in the above lists, “balls” will also be excluded automatically, then save your settings. The words that you will exclude here are mostly not important to you in the Index.
3. Generate your Index normally to move to Step3.
4. In Step3, please click the “Merge duplicates” button found in the Tools bar. This button will merge all duplicated words like “car” & “Car” that have same letters but different case sensitivity. Just click “Merge Now” button in the opened dialog. This will eliminate many words from the table.
5. In Step3, please click the “Merge plurals” button found in the Tools bar. This button will merge all words that have both singular and plural forms in the results table like “car” & “cars”. Just click “Merge Now” button in the opened dialog. This will eliminate many words from the table.
6. If you still see some unimportant words in Step3, you can simply right click this word(Or any group of words) and select “Ignore words”. That will add those words to an ignore list to automatically ignore them when indexing any other book later.
All the above suggestions should shrink the results you see in Step3.