Fill out the Less Than dialog box and choose a formatting style from the dropdown.
Select Highlight Cells Rules, then choose the rule that applies to your needs.
You can use wildcard characters to match multiple expressions. Often, you will need to add dollar signs ($) in front of letters and numbers in formulas so that the formatting is applied using absolute references as opposed to relative references (A1 to B1, A2 to B2). For example, if you want to make the whole row green if the value in column B is "Yes", write a formula like "=$B1="Yes"."