The answer here would be:
'Let's phone Paul, shall we?'
'That's a bad idea isn't it?'
In general for question tags, we repeat the auxiliary verb from the sentence. For example, if I say:
"You're from Ireland."
then my auxiliary verb is "be" ("You ARE from Ireland")
So when I make the question tag I say:
"You're from Ireland, aren't you?"
If my sentence doesn't have an auxiliary, then we usually use "do".
For example, if my sentence is:
"You like Metallica"
there's no auxiliary verb in this sentence, so when we make the tag it will be:
"You like Metallica, don't you?"
Try making tags for these sentences to practice:
You went to Germany, ________?
You've been to Germany, _________?
You live in France, ______?
You are from France, _______?
You enjoy Judo, ________?
You do Judo, ______?