What can you do before leaving your red eared slider home alone for a month?
They tend to eat tomatoes (small) make sure you slice it to his size and... Oh just bring someone to take care of him!
Tell Him that you Love him, and He will always be in Your thoughts....
Find someone to take care of him while you're gone unless you want a dead turtle.
I know you can leave reptiles without food for a long time, but having someone check on it and give it food is nice idea. You can also stock tank with fish and snails for it to eat.
get someone to watch him
their is nothing you can do except have someone take give of him. they aren't like fish where you can just put in a food block that lasts for a few weeks
Unless you want to spend a lot of cash on an automatic feeder and one of those cameras you can view from wherever that's always on, then tell a trustworthy friend to come over and feed him/ her every day