Kittens should be trained at an early age to use their litter box as its hard to teach them later in life.
Find an area that isn’t so out in the open. You’ll be surprised but like people cats need privacy and will be reluctant to go if someone is watching.
Make sure that they know where the litter tray is, it might be a while before they get used to it and try to stick to the same cat litter because they rely on their sense of smell.
If you wake up in the morning and you see that they have done a poo behind the TV make sure you tell them off they need to know the difference between right and wrong. It might deter them from doing it again in future.
I think doing a poo behind the TV, sofa, furniture and under the bed are the most common places that cats like to poo. You can buy deterrent sprays from pet shops that will stop them from coming on the couch and going to certain areas you don’t want them to.
The safest way to deter cats is to scatter citrus fruit skin they don’t like the smell, it’s better doing this in the garden as you’re sure they won’t mess up your plants and the fruit skin will disintegrate into the soil.
When it comes to indoors I’m not so sure you’d like the smell of citrus lingering, you’ll also have to throw away the skin every few days because they will dry up and this can be a nuisance.
I have found that for the first year to two years they will use a litter tray after that they get more adventurous as they explore and they will do it outside.
Make sure you have a cat flap it’s better because they won’t need to bother you when they want to go outside and you won’t find any nasty surprises when you get up in the morning.
Cat poo can be smelly but it is good for plants. If you find some near your flowers just dig a little hole and it’ll make your plants grow better.
It’s natural for cats to poo outside, so encourage them to do so.