I've managed to get my first frogspawn. I got two plates, one from a land frog and one from an UW frog. Now I've attracted 10 landfrogs, 3 UW frogs, 5 froglet and 7 tadpoles. I've also spotted 13 land frogs, 13 UW frog, 21 froglet and 22 tadpole. I guess that the 5th attracted UW frog will lay some frogspawn. Is that correct? How do I get the next frogspawn after that? Do I have to spot 15 UW frogs in another garden (and have at least 15 froglets and tadpoles)? I don't understand the mix between attracted and spotted. Please help!

Yes, you just need two more underwater frogs and the 5th will lay frogspawn :-)

If you have frogspawn and attract more tadpoles, then you can also attract more froglets. After you got more of them, you can also get more underwater frogs.
This is the basic principle of the lifecycle: For each tadpole attracted, one froglet will be attracted and this will lead to attract one underwater frog.

If you can attract land frogs depends on how many sets (tadpoles, froglets and underwater frogs) you have spotted in other gardens. Every 5th land frog spotted in the garden can lay frogspawn.