How do you "tag" someone else with a meme you were tagged on?
Tagging someone for a meme means that you are inviting another blogger to play along with a meme. To do it, you simply answer your meme as usual, then, at the bottom of that post write I tag this blogger, this blogger, and this blogger. (Of course, you'll need to change the words "this blogger" to an actual blog or blogger and then link to them.) Tags are usually done in threes, but there's no hard and fast rule here. You can even announce a generic tag and invite everyone who reads your blog to play along.
It's a good idea to send a private e-mail to the blogger(s) you've tagged to let them know you've tagged them.
As an etiquette aside, please do not be offended if you tag someone and they choose not to play. Sometimes people just don't like to do memes and it has nothing to do with you.