I want to replace the pebbles to larger white pebbles (20mm) with some stepping stones along the alleyway. How should I prep the area before putting the pebbles in? Can I just dig the stepping stone area and have it sit on mortar then fill the white pebbles on top on the existing one?
Welcome to the Bunnings Workshop community @alfiekino. It's terrific that you've joined us, and many thanks for your question about replacing pebbles.
The first question would be, what height level do you want this area to finish at? If you're happy to add the additional height and this area is already quite solid and compacted, you could leave the existing pebbles there and move straight onto laying the stepping stones on a mortar base. You should find these step-by-step guides useful: How to prepare and lay a base for pavers and How to lay pavers. You can then fill your pebbles around them.
Since this area has most likely already been prepped, this should be a relatively straightforward project. Make sure you check out the amazing Side yard paving and bike shelter project by @weekenddiyer for some inspiration before you get started.
I look forward to seeing your results. Please let me know if you have any questions.