Go to a store like Staples and buy a package of stationary with a colorful design around the edge. Something "baby-ish" is ideal.

OR

you can print out your own invitation stationary from a computer graphics program. Choose your own.

On your computer in the center of the stationary in a Large Font:

Little hands and little toes,
Tiny pink lips and a slightly pug nose.
Sweet baby soon to be here,
Can't wait to see you appear.

for_____________
date____________
time____________
place____________
____________
given by________
R.S.V.P.__________

OR

you can print, in a permanent marker, the above information on a baby onesie, baby t-shirt, or a cloth diaper. They would have to be mailed in a mailer envelope or taken around in person. Take them with you to the shower and give them to the new mom.