To insert a trendline on an Excel chart, you can follow these simple steps:

1. First, select the chart you want to add the trendline to.
2. Click the Chart Elements button which has a plus sign icon.
3. The Chart Elements window will appear. Check the box beside Trendline.
4. A new Trendline Options window will appear, allowing you to customize what type of trendline you would like to add.
5. You can also add a forecast, which will predict future values based on your Trendline.
6. Once you have finished customizing your trendline, click OK to apply the changes to your chart.