I found the specific steps for this activity in Rocks and Minerals. Although the book itself was a little advanced for my preschoolers, it had a lot of great information.
Keyword: DIY Rocks, Rock, Rocks
Materials
Sand
Pebbles
Glue
Disposable Cups
Instructions
Alternate adding layers of sand, glue, and pebbles until your cup is about one third full.
We waited and waited for over a week for our rocks to dry, but to our surprise they still weren't dry yet!
To speed things up, we separated the layers out and left them to dry over night.
The next day the pieces of rock were fully dry, and we were able to pick them up and examine them.
In a classroom setting each child’s rock will be unique, so be sure to leave time to observe and discuss similarities and differences between the rock formations!
Notes
Original post and additional photographs from Fantastic Fun and Learning can be found at https://www.fantasticfunandlearning.com/how-to-make-a-rock-with-kids.html