Okay, first have to start by saying i use the SheWee (not in a packy yet but working on it!)
I tried the medicine spoon, no go. Too small for me and got me frustrated to the point of not wanting to even practice. GoGirl works great but can't really be converted to put in a packy (too soft and bends too easily - need two hands, which looks ODD.)
I'm working on converting the SheWee into my packy since that seems to work best. I also made an STP (that could be put in a packy) out of a turkey baster bulb. Both work best for me and still working on modifying it even more.
1) I started standing in the tub, no water running so i could feel is anything leaked. For about a week i played around with positioning - where to put it, where not to, angle it needed to be to get the flow to actually GO and not back up in the thing....
Once i got it down i practiced aiming and move to the toilet (yes, still naked). Then i added underwear, then i added sweat pants, then jeans (still working on that one, though.)
2) I've always just been able to "go" in the shower standing. But i do find that i have to relax or i won't be able to do it by a toilet. It does take a minute to get things going.
It's more of a mental barrier than physical, i think, because for so long we've been told "girls don't stand to pee" (eye roll.) Now we have to work around it....
3) Slow the stream down by practicing even when you don't stand. If it's hard for you to do, then look for a bigger receptacle and work on stopping the flow on occasion to keep from overflowing.
I also go more often than i did before - if you don't have an urgent need, then the stream is more controllable.
4) Takes practice. It's also possible the receptacle is too small. You could also take a mirror and LOOK at where you place it....

I use SheWee which isn't small, but isn't as big as GoGirl. I found that the SheWee tucks right in even with my legs not spread wide and i usually only get overflow from the back (which is fixed by going slower or tilting a bit.)