OK I know some folks are sad that we don't carry Stampin' Up!'s word window punch here in the UK (yet!) but here's how to get the same effect using your scallop punch...ta, da!
First stamp your sentiment. I'm using the "Happy Halloween" stamp from the Occasional Greetings set, using Pumpkin Pie ink on Sahara Sand cardstock.

Then turn your scallop circle punch upside down and position it so that there is a scallop at each end of your word(s) and punch out.

Trim off the rest of the scallop by cutting straight across.

And attach to project of your choice...

I then used mine to decorate my paper pumpkin (inspired by US Stampin' Up! demo
Dawn Griffiths)