A useful method for changing standing up is to attach the tapes in known, tested positions, without relying on how the nappy is positioned on your body while in the act of changing. The aim is to tape every nappy into a replica of a well-fitted one that you have fine-tuned under better conditions.
If you can try putting one on lying down (ideally on your bed) you will have more control about where the tapes go and less of a fight with gravity. Put a nappy on with careful, symmetrical tape positioning. Get up, walk about, discover how that fitting works out in practice. Lie down, make any necessary adjustments, get up and try walking around again. Repeat until you have refined the taping position to give at least a reasonably comfortable fit, even if it's not perfect yet. Carefully note the tape positions on the front wings, using any available guide marks, distance from the edges etc. If you can pull the nappy down and take it off without untaping, you can take a careful look at the taping with it lying flat on the table.
Now try putting on a new nappy standing up, positioning the tapes exactly as they had been on the successful trial fitting. Don't worry if the nappy isn't yet in the right position on your body, just get front wings flattened out so that the tapes stick properly, and stick them in the right positions, starting with the bottom tapes. Once all four tapes are stuck, centralise and snuggle the nappy into the position it wants to be in, which should be a reconstruction of the original successful fitting. Once you have tried say five nappies fitted exactly the same way, if there are alterations you want to test, adjust the taping positions in small steps, being sure you know what the result of one adjustment has been before making another.
In theory you might get a slightly better fit by adjusting each nappy exactly to its own optimum fit, rather than replicating the trial one. But in practice, in a bathroom stall, you are likely to get a more consistent result, much more quickly, using the tested taping positions. I often use this method because I change a lot in public and workplace stalls. I can easily tape an ordinary nappy into a perfectly adequate fit in 12-15 seconds.