Start with a very simple search. Select the one or two most important criteria of your search. If this produces too many results, then click on additional criteria.
Use the reset button. Always reset after each search.
Text search. A text search should be short and simple. A search for "Rockefeller" will find both foundations, the Rockefeller Foundation and the Rockefeller Brothers Fund. A search for "Kenya" will find grants if they mention Kenya by name. A search for "fish" will also find "fisheries." A search for "conservation" will find many grants. But if you use too many words (e.g., "conservation of endangered black rhinos in Kenya"), your search will not find any results. Keep practicing until you understand how to make effective text searches.
Search progressively. If your first search is not successful, try again. Repeat until you find the best way to get the results you want.