What are the Sharia-compliant specifications for the depth at which a Muslim is buried? Can it be lined with stones? Is it permissible to prepare it before a person's death?

"Adopting the lahd (niche in the grave wall) is a Sunnah, and its description has been previously clarified. There is no harm in lining it or building it with stones if the earth is soft, except for stones that have been touched by fire, as it is disliked to introduce them into the grave. It is permissible for a person to dig his grave and prepare the lahd in it before his death if it is on his own property. However, in public cemeteries (al-masabilah), it is not permissible to dig it before it is needed, due to the restriction it imposes on others."