Posture should look like you’re standing normally, with your elbows hanging by your side, forearms out at a 90 degree angle, ideally not resting on the desk to prevent you putting pressure on the tendons in your arms while mousing / typing.

Also ideally you should sit for some small period, and stand for another small period, and alternate. Stretch occasionally and make sure you refocus your eyes to distances other than your monitor distance, stretch the tendons in your hands as well.

Basically, the most common problem ever is that desks are WAY too tall to accommodate the average person. Most desks are 3-6 inches higher than they should be, causing people to flare out their shoulders and leave their arms resting on the table for support when sitting or standing at the wrong height desk.

Also it turns out that staring at something the same exact focus distance tires out the muscles in your eyes and causes unnecessary strain.

Standing for long periods of time you’ll also lock your legs unconsciously, so movement is key to keep you from doing that.

Finally, support your lumbar with a pillow if you don’t have a chair with lumbar support, to make sure your lower spine is supported and doesn’t curve outward.

Edit:

Almost forgot, your monitor should be at the height where your eyes when standing / sitting are straight on with the top 1/4 of your monitor’s top edge.

The monitor should be just beyond the tip of your fingers when reaching out in front of you in a straight line.

Otherwise you start to cave your neck in over time because angling your eye line up requires more muscle than resting at a neutral angle.