How to make a specific plant depends on the plants you already have available for pollination as well as seeds you may have saved. So the simple way on how I can make it in one of my games may be different for you.

One simple cross is a Fragrant Maple with a Jalapa Maranta. If you look at the Pocket Guide You'll see that the number for a Fabled Lemonbush would be a 10/11 (each part has its own number.

The Fabled flower is in between the Jalapa and Fragrant (9 & 11) and the Lemonbush foliage is halfway between the Maranta and Maple (9 & 13) So when you cross a Jalapa Maranta with a Fragrant Maple, the resulting seed will produce a Fabled Lemonbush! Another cross that will work is a Fragrant Maranta with a Jalapa Maple for the same reason.

Moving further out from your goal, crossing a Roseceae Astera with a Fourpetal Ball Cactus (or Rosaceae Ball Cactus with a Fourpetal Astera will also work.

Those aren't the only pairs that will work, just find two flower numbers that add up to 19 or 20 and two foliage numbers that add up to 21 or 22 you'll get the plant you are looking for.

If you don't have pairs that will work, you will need to create one of those plants using the same process.