Many folks have complained (myself included) that there is not an easy way to add voice to Google slides. The Learnia may offer a solution. You can upload your slides (after you download them to powerpoint) and then add up to 15 minutes of narration.
Here’s a sample I created on signing into your school Google account. The site is designed for flipped classroom lessons, but I know you could create any type of tutorial you wanted. You can email the presentation or embed it.