You'll need to gather small river rocks the same thickness and are very smooth. You'll need to lay down a rubber base and pour a mud set floor and angle appropriately to drain. , set in the mud then grout. Usually a pro should install the floor.
Hi Kirsty, hope this is what you are looking for
You could set the river rocks into a waterproof grout or set in resin........
If you don’t already have a functioning shower pan in which to install the pebbles on top of, then to Kelli’s point, this is not a beginner’s project.
I assume that you already have an existing shower that you are wanting to update the floor diy on a low budget. If so, then education is the key starting point. There are a lot of resources you can look to. And yes, depending on your skill set, most beginners would hire a professional. After doing the research, if you feel you want to attempt the project, then you should decide the method your going to use and the material that will be needed for the project. One important note that hasn't been in any of the previous comments is: Make sure that before starting, you cover the drain hole, so that no debris or material can fall into the drain. Make sure that whatever method you use as your guide, that you have that information available while you are completing the project to use as a reference!
Do you have a pic of the floor?