In the early game, especially when playing support, I like to go in stealth just slightly on the enemy side of the lane, so, together with my adc, I can deal some relatively significant damage as the enemy adc enters the lane. It's generally a good idea to focus on stacking DH using your q->e combo (as in the early game this will deal around 50% of someone's hp, and the poison will then trigger dh, giving you a stack). Try to make the best use of your guerilla warfare, and don't forget to ward river to avoid ganks.

Once you hit level six, it's a good time to start shrooming up. I like to start off by protecting my lane with shrooms, then working my way up towards the dragon pit, walking back to lane every so often to poke. Once I start getting some cdr/hit level 11, I'll also start shrooming up around the enemy river entrance near raptors, then slowly make my way into the enemy bot side jungle quadrant. From this point onwards, I start terrorizing everyone in and around that jungle quadrant, running past my shrooms if I need to escape.

Make sure your shrooms are placed so that default pathing will most likely cause players to walk into them. This means placing them on outer terrain corners or bends of terrain, as when players walk around the corner, default pathing will take the shortest route, which is right up into the corner. It also helps to keep in mind how common it is to walk along a certain path, and try to place your shroom so that it interferes with multiple paths at once. Good examples of shroom placement are:

Right outside the corners of the dragon pit (these will activate not only when the jungler paths into the dragon pit, but also if they path along the river between mid and bot)

On the opposite corner of the terrain just below tribush, as this will trigger when your enemy laner walks back into lane.

On the corner across the river form tribush, as this will trigger when the enemy jungler ganks from river.

Places NOT to shroom up:

Lane bushes. They are very easy to avoid, and only trigger when enemies specifically try to path into them, which they won't.

River deathbush, for the same reason as the lane bushes

Inner corner of objectiver pits/jungle camps, as junglers will only path into these if they just did buff and need to walk around, whereas if you place the shrooms on the outercorner they will also trigger is someone paths around the pit/camp.