If the document is a PDF, it will generally open in Preview by default. But you can ensure that you open the document in Preview by right clicking on the file, selecting Open With, and then selecting Preview.
In the Preview menu, select View and click Show Markup Toolbar.
With the Markup Toolbar shown, find and click the Signature icon.
If you don't have any signatures captured, you'll see a box that says "Click Here to Begin". Naturally, you'll want to click there to begin.
As soon as you click, be prepared to use the trackpad to write your signature. You can just use your finger for this.
When you're finished, simply hit a key on the keyboard and it will record your signature.
To use your newly created signature, click on the Signature icon once again. Then click on your signature and it will insert your signature onto the page. Once on the page you can place it where you'd like and resize it to fit.
Because your MacBook is so important for all that you do in a day, it is important to take good care of your Mac battery, monitor your Mac's battery health, and check the cycle count of your Mac's battery when to determine if your Mac needs a new battery. This might sound like a lot of maintenance, but checking your Mac's cycle count is an easy way to determine if your Mac needs a new battery, and it's actually incredibly simple. A battery's maximum cycle count is how many charge cycles your Mac can complete before the battery is no longer able to perform and will need replacing. In this guide, I show you how to check your Mac battery's cycle count and determine if your battery needs to be replaced.