To create your own select Design Equation Ink Equation. Under Table Tools on the Layout tab in the Data group click the Formula button. Depending on the size of your screen you may have to click the Data button first.
To use a built-in formula select Design Equation.
Get the complete tutorial FREE. You can click Write button and use your mouse or writing pad to write the formula here. In its wisdom Word makes an educated guess about what you want the formula to do and places a formula in the Formula box. Select Insert Equation or press Alt.