Can we pray Sunnah prayer after the respective wajib prayer? And can we pray qazza of Sunnat prayer? And how do we pray sitting sunnat prayer after Isha?

1. Sunnah prayers are prayed before Fajr, Zhuhr and Asr, and after magrib and Isha

2. Yes if you want to. There is nothing wrong and it’s rewarding

3. Two Rakat sitting is equivalent to one rakat standing. You make your takbir while silting, recite both Fatiha and Sura, bend half way for ruku and full to the ground for sujud.