Writing Nenga Jou

Please help me! I want to write Nenga Jou to my workmates but have no idea how to do it. I bought some Nenga Jou postcards that already have messages printed on the front. On the back there is room for an address and I assumed I should write a short message underneath, as one would on a regular postcard. But my colleague told me people in Japan don`t write personalised messages, they just write the recipient`s address only. Is this true? If so, am I supposed to write my name on it somewhere so they know who sent it? And if so, where should I put my name? Thanks!

There seems to be some confusion between you and your friend.
(1) Normally, you write the address and name and postal code of recipient and those of sender on the stamp (often already printed) side.
(2) You are supposed to write your messages and greetings on another side of the card.
(3) Only when the side without stamp is photograph (where a pen does not work) or there's too many messages and does not have enough space left, you can write on the stamp side of the card. But this is a rare case, normally you should follow (1) and (2).

(Note: sometimes people print sender's name and address on the non-stamp side of the card.)

Thank you so much, this clarifies a lot! One more question, how long should my message be on the front? The cards I purchased already have messages printed on them and there's only room to write two or three sentences, is this sufficient?

what you say "front" is the back side in Japanese.

front side(address side)
http://livedoor.blogimg.jp/news30over/imgs/f/c/fca55650.png

back side
http://www.wanpagu.com/sozai173.html

if you want to write many messages, you may buy a nengajyo of enough empty space on the back side.
http://botan.rc8.jp/wp-content/uploads/2012/01/nenga1.jpg

To write two or three sentences is enough!
Typically it's 昨年はお世話になりました。今年も宜しくお願い致します or something only.
(Thank you for your cooperation and support last year. I look forward to your continuing support this year.)
Because Japanese people think that printed message only is heartless, so add hand-lettered message a bit.