Level 1

Ideal:

At level 1, you want to ideally start with Pale Cascade. It deals roughly the same damage as Crescent Strike and gives you a shield. If you are invaded, it is acceptable to grab Crescent Strike if you plan to contest it, otherwise just grab Pale Cascade.

Level 1-5

Ideal:

Deviant 1:

Deviant 2:

Diana does not have a particularly deadly gank pre 6, so it would be ideal Moonfall is picked up at level 5. When you hit level 3 and 4 look at the nearby lanes and see if you can apply pressure, if gank the lane and skill Moonfall if you are able to get in range. It is completely acceptable to Flash + Moonfall if you feel like you can kill your enemy because if you can make your opponent Flash it's easier to successfully gank the lane Post-level 6 when you have Lunar Rush.

Level 6+

With Lunar Rush Diana's kill potential becomes incredibly high. Look for lanes who have low health opponents, or allies with strong CC. Don't force yourself to gank, the more gold you can accrue from farming, the stronger your presence will be. You have high control on whether or not to Lunar Rush in, watch yourself, analyze the situation, and avoid being too tunnel-visioned.