Open your Google Docs document
Place the cursor where you want the fraction
Type the numerator, then a slash, then the denominator, such as 1/2
Go to Insert
Select Equation
Type the fraction using the equation toolbar or format, such as frac{1}{2}
Use the keyboard shortcut Ctrl + / if you want to insert a comment, not a fraction
Use the special characters menu if you need a fraction symbol like ½
Copy and paste a fraction from another source if needed