Sorry if this has been asked before, but....

Probably the others told him about all those stories and then he tells them to his kids.It's a sitcom ,don't take it too seriously.

Like the previous reply says, It's a TV show. It doesn't have to be 100% reliable. In that case the show would have lasted only a few episodes because it's impossible for Ted to remember everything word by word.

Those scenes are how he pictured them, I think (though it could also be how his kids picture them, or both). He is told enough detail by his friends to "set the scene" and then fills in the gaps himself, with his imagination. He basically knows the gist of what happened, but has his own version of how it happened, so while Ted's stories are accurate unless stated otherwise, they are basically his "idea" of how that scene went down.

There are certain parts where Ted admits that this isn't how this event necessarily happened, but he just has to take the other characters word for it, and even when he doesn't say this, there are hints that he is just filling in the gaps; some of the more unrealistic looking scenes happen when Ted isn't there (and when he is, it can be explained as Ted exaggerating, or remembering wrong for another reason, like he "ate a sandwich" that day, or something), and another thing that has been brought up at least twice, is that certain locations resemble locations from other television shows - Ted wasn't in those scenes, so it's possible that he mentally designed the layout of that building when he heard the story, and used something from another TV show to do it.

Because otherwise it would be one hell of a boring tv show :)
We're not supposed to think about it - just like we're not supposed to think about the fact, that in "real time" it would've taken 3 days with Ted talking non-stop, to tell all of this.