All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We have already issued Fataawa 83175 and 82692 about how and where to place the hands during the prayer. It is to put the right hand over the left hand on the wrist on one's chest, as described by Yahya one of the narrators.
Putting the right hand over the left hand during the prayer is recommended in every prayer whether obligatory or supererogatory.
Whoever performs the prayer while keeping his hands down or placing them below his naval, his prayer is correct and acceptable, because putting the hands on the chest is only a recommended act during the prayer and not an obligation.
As regards the Tasbeeh prayer, please refer to Fataawa 86275, 83877 and 83600.
With regard to the mistakes which you used to make while performing the prayer, and which were not in accordance with the Sunnah, then if these mistakes are like abandoning the Sunan and the desirable acts in the prayer, like putting the hands on the chest or not raising them when bowing or when standing from the bowing position, then these mistakes do not invalidate the prayer and the prayer is correct and acceptable. However, if these mistakes are neglecting some obligations like reciting Al-Faatihah, or not observing the Tuma'neenah (this is defined in Fatwa 90057) when performing the pillars of the prayer, then you should make up the prayers in which you did not perform the pillars properly.
For more benefit on making up many missed prayers, please refer to Fataawa 86918 and 83727.
Finally, we advise you not to despair of the Mercy and Forgiveness of Allaah, as His Forgiveness is vast and He Accepts and is happy about the repentance of His Slave. All what a person is required to do, is to sincerely repent to Allaah from the mistakes and sins of the past, and to perform good deeds in the future