I was a flapper for Halloween this year and, not being one to waste a costume, I also wore my flapper outfit to a 20s Jazz Banquet I went to. Here's a simple outline of tips for 20s makeup.
For the eyes:
Dark kohl eyeliner- Apply around eye twice. Extend upper line past the eye a little bit.
Darker neutral eye shadows, especially browns. If desired, use white eyeshadow as a base before applying the brown eye shadow.
Big eyelashes - some people prefer using fake eyelashes, I just used black mascara.
For the cheeks:
The "pale" look was very in. To achieve this I used:
Liquid foundation - I just used a lot of it because I'm pretty pale already
Powder - This just gets ride of shine
Blush - In a pink/rose color
For the lips:
I don't own lip liner, so I couldn't use that, but you can to create fuller lips.
Red Lipstick with Clear Lip Gloss - To create a dark red look for the lips OR
Red Colored Lip Gloss - Gets the same effect, just depends on what you already own