How do I make stepping stones or islands in the middle of a pool?

Hey sims family! 
I’ve finally had enough of not being able to make stepping stones or an island in the middle of a pool, I’ve watched tutorials, tried everything and still nothing works for me. 
there’s multiple houses you can download that have both and yet it seems impossible! 
HOW DO THEY DO IT?!? 

Here’s some reference photos for everyone so we all know what I’m talking about and for others with the same issues as me aha. 

If anyone can help please give me step by step instructions please & thank you!!