You have to offer lots of voluntary prayers and sincerely repent for the missed prayers.

I reached puberty at a very early age, and when I was younger I used to regularly miss Fajr or Isha Salah. (For a few years) .However when I got older I began to pray 5 times regularly Alhamdulillah. My question is, is it sufficient to pray 2 rakahs and seek forgiveness for those prayers or do I have to make them all up? This is a bit impractical as I do not know how many I missed.