When in the Safari app on a recipe that you would like to Yum, click on the share icon that looks like a box with an arrow coming out of the top. Scroll down to Yum. If you do not see Yum as an option, scroll to down and tap on "Edit Actions". Make sure that the Yum option is turned on.
Once that is turned on, you should always see the Yum option in that bar and just tap on that for it to be added to your recipe box.
Don't see Yum in your list of action options? Try restarting your iOS device.