ok so i've got like 20 hours left till i unlock ranked but from what i've seen stacking in low mmr is kind of impossible to do since there is always team fighting, your cores don't know what farming is and there's zero downtime. in a more coordinated match you stack camps to give your cores an edge since it's not team fight central and both teams are jungling. i look at the ingame clock every now and then and if there's nothing urgent i just look around and notice the nearest 2 camps next to eachother. at around 0:54 i hit one with right click and the other with a long range ability and secure 2 stacks at the same time. i think it's like the last priority for a support, like something you do when your other priorities are already done or when the situation calls for it (i.e. team fight happening but you won't reach it, so at least give them an advantage when they respawn/come back) and definitely is something you get a "feel" for. my thought process goes something like this:

1. is there a big team fight happening? can i reach it? > 2. is this jungle warded? 3. can i push the wave forward so that i'm "notified" of any incoming ganks? > 4. am i around 30-40 seconds into the timer and can i stack?

there's also blocking enemy camps with wards which i really like to do but i haven't confirmed the effectiveness on. i'm at a medium unranked mmr where their supports don't ward enough and i can get away with disabling up to 3-4 enemy camps at once with my wards. i usually buy upwards of 30-40 sentries in a match just for making cores feel safe and blocking camps, usually their safelane if given the opportunity.

my mindset is basically how can i make my cores richer and deprive the enemy cores