What it is
Rowing machines are widely spread and used in many cases for cardio training.
But there are just a few rowing machines which are equipped with an additional backrest, which is necessary if you have not enough core strength yet to do rowing without. Another possibility is just to buy a first or second hand rowing machine and mount an ordinary seat or backrest on it. We have seen this already several times - it is working perfectly, especially when you consider that we are using the rowing machine not in a high performance way but just to have some movement and load on the joints and bones and hopefully some additional fibers getting stronger or reconnected in our spine.
How to do it
Depends on your actual functionality - If you are at the moment not able to use your legs to do the movement yourself, just help with your arms by pushing on your knees and help making the movement this way. In this case of course you can not use the arms to additionally use the handle of the rowing machine at the same time. If you have a helping hand friend, family member, therapist you can use them to help you with the legs and then you can use the handle.
While you are doing the movement try to think about the movement you are doing and use the right muscles in your mind.
What to consider
Just to mention, don't forget the additional pillow to save your butt.