Treat them like little girls and don't take them seriously. That's it. It will come naturally with this mindset.
I'll give you guys some examples that I use.. Remember to do it in a playful way with a smile.
When she asks for something, don't give it to her right away. Pretend to give it to her then take it away. Repeat.
Play 'The repeating game' (The game where you repeat everything the person says straight after they say it)
When she says something, say ''What?'' repeat to taste.
Play the 'Stop hitting yourself' game
Play a game of tag
Put things in her hair
Put her hand on you and say, ''Stop touching me'' (remember, be playful)
''I know you are but what am I''
Say ''What's that on your shirt?'' then snap your fingers under her chin when she looks.
When you're sitting/laying on the couch together, tell her she is on your half of the couch and proceed to gently nudge her away.
When you're walking together, push against her gently and pretend to try and run her into things.
Give her a hug and then pat her on the head like a dog and say ''Good girl!!!''
Call her a dork.
Tell her ''You know, you're almost cute enough to get away with saying that....ALMOST''
If you're drinking with her, look into her eyes and with a straight face say to her.. ''You know what? Maybe it's just the beer talking, but I really................love....................................beer.
Tell her ''It's a good thing you're still pretty'' after she does something dumb
And my personal favourite - Raise your hand for a high five then when she lifts her hand, drop yours and say ''who's gay''
These are just some that I have used but once again, just get in the right mindset and it will come to you easily.