Easiest way is to use jack stands or blocks etc & a creeper... Which is how I've always done it.
Proper way would be to put the chassis on a rotisserie, flip it upside down & do it standing. Or horizontal etc... Ideally, you'd grind the rail/floor to find all the spot welds to drill out & actually remove & replace the whole rail properly vs capping the rails like most people do.
But don't worry about doing it upside down... These things called welding helmets will prevent you from getting mig slag in the face
Now if you were under an aluminum boat with a tig the risk of a runny puddle does come into play