Hard way: print out what you want to write on the cake and mark lines with pins. Then, imprint the pattern onto your crusted buttercream. Some people use clear gel also.
Easy way: use a cake projector!
Tappits are so cute, but beautifully hand lettered cakes look so elegant to me. Unfortunately, my hand writing leaves a lot to be desired, so I've been experimenting with different styles and fonts. I made up some "practice" buttercream using just shortening, powdered sugar, and water....several times a week I practice making different flowers, then I practice writing. I'm really improving! The icing can be scraped back into an airtight container and used for many months. I got the idea from Serious Cakes.
There's an old book (now out of print, I believe) called Writing in Icing by Mary Ford. I checked it out of my library and copied some of the pages. It has some helpful info to get you started on the right foot, then gives pages and pages of examples of different styles of writing. Maybe ebay has some copies?
I agree with Leah! I also love my cricut cake for lettering...
Easy way: use a cake projector!
That's what I am talking about!!! 
Tappits and the cricut are great if you have them. Most likely, if you are just starting out, you don't.
This is a tutorial I made a long time ago before I obtained the "fancy" toys:
http://home-and-garden.webshots.com/album/265808017nRgprD
It makes writing on a cake a snap.
I can't pipe text to save myself (and nothing ruins an otherwise perfect cake quicker than botched piped text!) so I LLLOOOVE my Tappits - once you get the hang of them it's really easy to make PERFECT lettering. I've just bought a set of Windsor Clixstix and they are brilliant too - they're quicker and easier than tappits. ...Sorry I'm no help on piping!
Wilton sells letter presses that I use a LOT (my paper writing is kinda pretty but my cake writin really sucks!). Another tip is to move your whole arm when writing. Don't move just your wrist, like you do when using a pen or pencil, but move your whole arm in a nice flow.
Print out some large font phrases and practice piping the icing on the top of them until you get the feel for the icing flow and the arm movement.
Love my tappits and just bought hte cricut mini over Thanksgiving weekend, but havent had a chance to sit down with it yet. Hopefully with both of those I wont have to write too much with buttercream.. cause I stink at that!!
You either got it, or you dont. Just my opinion. I detest writing on cakes. make sure you have good crusting buttercream so once it dries if it sucks you can pick it off with a tooth pick and start over
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