Start out with straps in more in board position until you get proficient. Make sure you lift your heel (kinda like a chick stepping into high heel

) and make sure you use toe pressure to keep the board riding flat... when you lift one foot you can tend to rock back on the heel bringing the rail closer to the water which causes heel to catch and also sends you upwind. Dont look down this will also cause you to rock onto heels and do the same.
Harness, I always went into footstraps out of harness to give me some chicken leeway- easier to drop sail than get catapulted in harness. It is actually easier though to use the harness make sure you take your weight outboard then what you do with your feet matters less. It is the surging change in power that will cause you to get catapulted anyway, using the harness gets you more committed and power more even. Go to get into footstraps on a beam reach, broad reach you are accelerating too fast, pointing upwind you leave yourself no room if you do stuff it and screw into the wind further.
On a light-moderate wind day just practice, hook into the harness and sail with only your back foot on the board and then with your front foot in the strap, once you learn to trim the board flat and keep the heel clear of the chop you can sail for ages like that and do whatever you want with the other foot.
Also try to use only one or two boards so strap placement becomes natural, constantly swapping boards can stuff you up a bit. Oh and use good quality footstraps that sit up, I love the neoprene pryde straps. Most of the new ones are pretty good though.