I've just found a better way.
1. Click on the "Type Text" Button (under the "Home" ribbon)
2. Click in your document where you want the tick to go
3. Go to the top let of screen - you will be able to change the font and the size from dropdown lists. Change the font to "Webdings", change the size to whatever you want (18 works well for my purpose)
4. type lower case a for the tick, lower case r for an X (or you could just type an X of course....).