I finally did the one-hand plank yesterday and I was able to hold it for this picture. It took me around 3 months to do it, being introduced to it pre-stellar and now I had to go and do that almost every week.
To do the one hand plank, You gotta 1) grip with your thighs 2) get your strong hand at the bottom (in planks, it's my left hand) 3) arch your back and look back 4) keep the feet down and tight so you don't end up in a cross-ankle release). The pointers aren't enumerated in order, but they're just key points.
For the most important part, one-hand planks are like big chances. You just gotta do it. If you hesitate, you loosen your grip and fall (It happened to me). Once you're there, go na.