Sleeping after Fajr

Assalamu alaikum,Respected Mufti Sahab
While standing in the saff for Namaz, should the saff be straightened/ lined by making the the toes in the front in line will all the mussallies or should the saff be straightened by straightening the heels from the back of the mussallies. There is confusion and some people stand forward and some people stand backward and in this way the saff is not straight. What is the correct procedure?
Jazzak Allah!