Pray for good weather. Prepare yourself with warm clothes. It can be cold in the shadow !
To do landscape paintings outdoors, you need an easel where you can alter the length of the easels legs.
If you put a spirit level ( bubble level ) on the top of your canvas and carefully get it horizontally, you can trust the angles to be 90 degree sharp.
This is very useful if you for example have a treebranch that is difficult to trace the angle of.
Below you can see some of my attempts in this field. Most of the paintings have taken two to three days to complete and are painted on the spot.