If you are talking about small pull out tags, I very often include these on my layouts, most commonly underneath a photo.
The most important thing to remember is to create a pocket on the back of the photo before adhering the photo on the layout. Seal three sides and leave one side open for the tag. Make the pocket a little wider than the tag but less deep (not as tall as the tag). That way, the tag can't slide all the way in and get lost.
I usually add a ribbon to the tag so that it's clear to the viewer that he/she should pull out the tag to read the hidden journaling.
In your design, you should consider the space that the peeking-out part of the tag will take on your page and design that as part of the page. In other words, don't think of the page design as having a completely hidden tag, but rather a slightly poking out one.
Here's an example of a tag in a pocket under a photo.
Snowbody Melts My Heart By IzzyB
One more, though this tag isn't very hidden - at all! But it does slide into a pocket under the photo.
2 Months Old By IzzyB