“One must have kavanah”: for the main reason they established saying Tehillah LeDavid every day is for the sake of that pasuk, in which one mentions the praise of HaKadosh Baruch Hu, that He watches over His creatures and provides for them.

“Again”: see the Chayei Adam, who brings in the name of the Levush that one must say [it] from the pasuk “You open” to the end of the psalm, in order. And if he did not remember until he had already said other psalms, and he has no time to go back, even so he should say, after the prayer, from the pasuk “You open” to the end of the psalm.