Bismillah
salam
Thank u curious, of course I understand, we all work for living and cannt spend all day on here.
Anyway, as such we can say that prayer including making the sign of the cross was not certainly made by jesus or his followers. Simply because the corss at that time didnt gain the position it has for being a symbole of sacrifice and suffering. Who decided then on the form of prayer? and as u said, I mean who actually commanded you as a christian how to worship your God?
In other words, prayer's root, apart of course from the passage which goes like our father in heaven...etc, is not established by a divine passage. Am i not right? I m aware that the word prayer to u includes just making supplications.