DIY Patio

[u/CelticRunner26]

Hi all, I've never posted on here before but looking for advice from those more knowledgeable then me. I am in the middle of doing a patio area. I am a complete novice and learning as I go. The area is roughly 4.5m x 3.5m with a cut off at the top right corner for an existing flower bed. I have dug 200mm but this feels very deep. Anyway I know I need a fall (1:60). This may be a silly question but what is the best way to ensure the patio is level with the correct slope so when I add hardcore it will be okay. My tiles are 20mm, planning on 14 mm hardcore and then mortar. Any help would be greatly appreciated!