Selam what is the proper way of positioning feet during sitting/ kneeling aspect of prayer. I see some websites instruct to sit on the L foot and right foot is erect. I like to sit on top of my heel while my feet are next to one another keeling of course. It is easier position for me to maintain because sometimes I have pain if I hyperextend my foot. Is my prayer valid or do I have to follow what those websites instruct? thx
Alaikumassalam wa rahmatullah,
The way of sitting you described is how the Prophet 
is said to have sat when praying. However, there is no hadith where the Prophet commands people to sit in this way. It is a way of copying the Prophet’s way of sitting, and we do not know with absolute certainty if it is the only correct way of sitting. So while we can consider it the recommended way of sitting, if you have difficulty sitting in that position then you can sit in whatever way is easiest for you. We know that Ibn Umar 
did not sit in the recommended way because it caused him pain due to an injury he had suffered in the past.
Alaikumassalam,
Brother I would advise you to sit like that as the Prophet (s.) forbids sitting on the Hebels in one Hadith in Sahih Muslim :
‘A’isha reported:
The Messenger of Allah (ﷺ) used to begin prayer with takbir (saying Allahu Akbar) and the recitation: “Praise be to Allah, the Lord of the Universe.” When he bowed he neither kept his head up nor bent it down, but kept it between these extremes; when he raised his head after bowing he did not prostrate himself till he had stood erect; when he raised his head after prostration he did not prostrate himself again till he sat up. At the end of every two rak’ahs he recited the tahiyya; and he used to place his left foot flat (on the ground) and raise up the right; he prohibited the devil’s way of sitting on the heels, and he forbade people to spread out their arms like a wild beast. And he used to finish the prayer with the taslim.
Reference : Sahih Muslim 498
In-book reference : Book 4, Hadith 272
USC-MSA web (English) reference
The Brother that commented before mentioned Ibn Umar (r.) not sitting this way due to an injury and he is absolutely correct, but if you are not injured you should do your best and avoid sitting on your heels because of the hadith mentioned above.
And Allah knows best.