Hello @Zeblan and a very warm welcome to the Community Forum 
As @Debbie-L mentioned, you can reply to them via the messages from when they applied to sit for you. For future reference, once you confirm a sitter the system will automatically send out a decline notification to all the other applicants so it’s always best to send them a note before confirming your chosen sitter as I am sure you meant to do. Thank you for being a thoughtful homeowner with regards to sending a polite note to the unsuccessful applicants, it is always much nicer as a sitter to receive a written note rather than just receive the automated decline message.
For reference as a member, you can also add your TrustedHousesitters profile on to your Forum profile by following the attached link, should you wish. This will then enable others to offer you helpful advice and feedback and will also give your listing extra exposure!
If you are unsure how to do this, or if you have any questions at all, do not hesitate to ask, we are all more than happy to help.
Also, remember you can use the spyglass symbol at the top of the page next to the TrustedHousesitters logo and search for any questions that you may have as they have most likely already been answered on here and of course if not then please just ask! 