Using Yazzy is very simple. From the app, you can choose what app to create a fake screenshot for. There are 12 in total, including the messaging apps mentioned before, and others like Google Translator or Ask.fm. You just have to tap the app's icon and you can start making your fake screenshot.
Depending on which app you're using, Yazzy lets you use specific tools, although generally you can create a fake conversation with anyone in just a minute or two. When you're done creating this fake screenshot, you just have to save it and send it to your friends.
Yazzy is a fun app that gives good results, letting you trick your friends with fake screenshots. In the app settings, you can also deactivate the watermark.
Requires Android 4.0 or higher