Welcome to the forum Mazikeen.
The easiest way is to not let them on the furniture from the beginning but there are things to try. I had THIS
site bookmarked but here is what they recommend:
First you are going to quietly lead your dog to the sofa or other article of furniture you don’t want him on.
Click and treat if he keeps all four feet on the ground.
If he puts a foot, or other body part on the sofa, use a kind voice tell him “off” and use the leash to help him get all four feet on the ground.
Now continue to play this game.
Do not (at first) have your dog sit or lay down this avoids the problem and the learning.* Just because he is laying down on command, doesn’t mean he understands that THAT keeps him off of the sofa.
You want him to explore and learn what you want.
Continue doing this until he is comfortable with all of himself on the ground.
Now you are going to coax him up, but you can’t cheat and use his name.
You can pat the sofa, you can put his toys up on the sofa, you can put food up on the sofa.
You WANT him to make a mistake.* Yes I realize that sounds funny, but in order to learn he has to make the mistake of getting on the furniture in front of you; and he needs to know that no matter what you don’t want him up there.
When he does make a mistake, you aren’t going to blow up and get angry… remember you expect it; you want it!* So when he does tell him “Off” in a kind, yet slightly commanding voice and click and treat him for getting off use his leash if you need.
Remember to use OFF and not DOWN.* I can’t tell you how many times my husband says DOWN when the dogs are laying in his spot in the bed (ha ha) and they wiggle around and put their heads down because they don’t know he means OFF.
If you use the wrong command it is YOUR fault not your dogs!
Now it is time to switch up the furniture and play the same game all around your living room and bedroom, or wherever else; being sure to reward for a job well done.
Your dog should pick up on this pretty quick.
And, later if he makes a mistake you can use a short tab leash to help him get off and then take him to his dog bed and command him down.
Be consistent and make sure everyone else is!
Set him up for success by buying him cozy places to lay.
And TEACH him what you want by utilizing his leash and rewarding him for a job well done!