When you want to spend directly from a budgeting Jar (i.e. a Jar not connected to a merchant), go to your Home screen, long press on the Jar and tap ‘Link to card’ - you will know which Jar is linked because it appears at the top of your Home screen with the card in it.
To spend directly from a Merchant Jar, use your HyperJar card at the till or online with the merchant and money automatically comes out of the right Jar.
If you have multiple Jars opened with a merchant featured on the app and you’re ready to spend, you need to choose which one you want the money to come from. For example, you might have a Christmas notonthehighstreet Jar and a Birthdays notonthehighstreet Jar. The Jar that is linked will be listed under the Auto for payments section of your Home screen, and the one that isn't will be listed under Inactive. You can change between them easily by long pressing and tapping Link to my card.