Once you have an access token, the hard part is done! You can use the access token in either a call in a client library or you can make a direct HTTP POST request to our checkins/add endpoint. The URI should look like, before adding any other optional parameters:
https://api.foursquare.com/v2/checkins/add?oauth_token=ACCESS_TOKEN&venueId=VENUE_ID&v=20130822
Note that Foursquare also released native auth for iOS, which can make getting the access token even easier without having to use a UIWebView.