Launch the Facebook app on your phone. Select the "Check-In" button at the top right. Select your location and add your post message.
Click the "Add to your post" button and select "Tag Friends".
Once you select the friend or friends you want to tag, select "Post". You should then see the post on your newsfeed with the tagged person or people included.
Every person tagged in a check-in will count as an additional check-in and an additional donation. For more specific steps on how to check-in on your phone, check out this article.