my truck doesn't have the tailgate step so I picked out a basic step ladder at a local hardware store. how should I mount this so it doesn't move all over the place.

 

some kind of clamps / brackets on the side of the bed? or on the tailgate itself, although that could get in the way.
either L-brackets or those half-round that "snap" around the top round handle?

 

I would consider mounting somehow utilizing tire carrier. Anything left in the back is bait for thieves.

 

How about some bungee straps to the metal brackets on side of bed

 

i like the tailgate idea but how would i mount it to the tailgate?

it looks like this folded in its storage position

 

i was thinking just laid down / held in place with some kind of brackets, but then you have to remove it, put it down and unfold it to use.
now if you wanna get real fancy, I guess you'd need some kind of offset hinged bracket that let you "fold" it open and extend it down to the ground, but that gets tricky.
hate to say that now, but i think you could have found a tailgate-specific ladder of some sort that would have already had all of this figured out
just one example: https://www.realtruck.com/traxion-tailgate-ladder/

 

 

in my 2012 I tried many different non permanent attachments. Ended up just leaving it in the back and carried a telescopic hook in the bed. Even though I have the stairs in the 2016 I still keep the hook back there. Good luck.

 

I went with my step ladder because of the large foot area, I can get in and out of the bed with ease even with hands full

 

^ cant argue with that, I love my tailgate step for that exact reason...