well, I'd refuse to feed in a bathroom unless your mom is in there eating her lunch too, but that rant aside, it's going to be easiest to get the hang of sling nursing probably at home (or another comfortable environment) & when she's hungry, but not really hungry, so either when she's just starting to be hungry or after being fed a bit, if she's too hungry she'll have no patience with trying something new. If possible, try a position similar to a familiar nursing position. For example, if you nurse her in a cradle position or on a nursing pillow, then a cradle hold sling position will be a bit familiar to her.
Now, if you aren't having any luck with the above, you might try having the sling on & around you both, but very loose/not tight enough to be carrying her (or for her to notice) while sitting or lying to nurse. Once she's nursing or has fallen asleep, try tightening up the sling & getting up with her (but make sure you've tightened it enough & she's snug). This method would be easiest if you sat to nurse (rather than lying down) & is a bit trickier than the other, so I'd try it only if you have no luck the other way.
Good luck & feel free to ask more questions, and please don't let anyone make you feel like you need to feed your babe in an unsanitary public restroom.