Both females were trained while we lived in an apartment, but I just went downstairs.
I never trained the pups for a specific area in the house because imo it is much harder to get them outside from that point. And yes...It meant a lot of time going downstairs

And never said anything excepts "oops" when they did something in the house. Just picked them up again to go outside.
And training outside was just a couple of square meters...Not going for a walk! There is absolutely no point in taking them for a walk because you'll get exactly when Debby said: They will do it the minute you come home...
Just put them on the grass, if necessary take a chair downstairs, ans wait there untill he does something. Then praise him and take him upstairs immediately. Thats how he learns the connection.