How can I tile over an existing kitchen backsplash ?

Is is better to remove the old backsplash. It will not take long to do. Make sure you put paper under the project for easier cleanup. There are you tube videos available. Video is also available to tile over existing tile. My past experience for you is to think about the project, think of how tiling over old tile is going to leave a unsightly ridge or gap that is noticeable. It’s better to do it right or don’t do it at all. You don’t say how big your project is. Take a picture and go to your big box store and check with the the tile professional there. They can recommend products to cover your project area. There are some beautiful tile samples now in glass, wood, stone, etc.

https://homeguides.sfgate.com/cover-ceramic-wall-kitchen-tiles-backsplashes-42766.html

https://www.bobvila.com/articles/can-you-tile-over-tile/

https://myoldcountryhouse.com/yesyou-can-tile-over-tile/

https://www.thesmarttiles.com/en_gb/inspiration/cat/tips-and-advice/

https://diy.stackexchange.com/questions/39909/what-is-the-easiest-way-to-replace-an-existing-backsplash

Good luck Renee on your project.

I recommend that you remove the old one. You need a smooth, flat base for the new one to stick.

The best method for tiling a backsplash is to install cement board first. Then lay the tile with a pre-mixed adhesive. While waiting for it to set, we usually put an occasional small screw to hold in place and prevent sliding. Once it's set, you can grout then seal once grout is set.

However, here's a Hometalk project that uses a mat instead of the cement board. I have no experience with how this lasts but this might be the route for you.

https://www.hometalk.com/diy/kitchen-dining/backsplashes/arabesque-blue-tile-backsplash-using-an-adhesive-mat-7650501