Disclaimer: As of season 4 blossom builds have really fallen off. They have no support from our tierset at all and face considerable mana problems, added on to this the current playstyle of echo builds that consist of mostly echoing rev are also significantly easier to play which means even the easiness of blossom isn't really a good consideration. I heavily recommend playing echo builds instead

The basics

blossom builds are really different from the rest of the preservation gameplay, they are only really recomended for raid and feature less raid cooldowns than echo and a somewhat worse healing profile. Their strong points are being incredibly easy to play, having a decent amount of passive damage and making you incredibly tanky via panacea

The central goal of blossom builds is generating as many bursts as you possibly can, in order to cast more blossoms

af

This talent is one of the most important parts of blossom builds. Each time you cast blossom your next lf will cast 40% faster, and this lf can proc burst so you can cast an extra free-blossom. Note that burst blossoms are free of both essence and mana.

As both blossom and healing with lfs cost a lot of mana, constantly casting quick lfs on the boss to proc burst is how you are able to cast blossoms constantly through a fight without going oom, and most of your gameplay will be one blossom followed by one lf on the boss and repeat.

reso and rev

On a blossom build you aren't running timelord, but you do have reso which means your ta will put lower-power echoes on three friendly players. Healing spells won't do much into these echoes as their power is very low but any rev you cast will get echoed and each rev will have its own chance to proc burst. So casting ta and then using rev gives you as much chance of proccing burst as casting four revs on its own would.

ta on it's own still costs a high amount of mana, so this is a tool you can use to fill your resources back up once you run completely dry, but it should be using sparingly and it is the first thing you cut back on if you start having mana problems.

Positioning and targeting

Dropping a lot of blossoms on the same clump of players everytime would result in a lot of overhealing, and casting them on random injured people in your raid is terrible because you don't know where they are positioned and if they have enough players around them. While the ideal situation for blossoms is having the whole raid stacked in a single point this won't always be the case, and you have to adapt your targeting accordingly.

The most effective way to target your blossom, is to always stack yourself with the ranged group, and pick a melee player you trust that is always stacked on the melee group, this could be the tank but some mechanics require the tank to be away from the group so you have to be careful of that, or a specific melee player you trust to have good positioning. When you cast your blossom, you would alternate between casting them on yourself to heal the ranged and casting them on your trusted melee to heal the melee.

Another way to target blossom is the model-mouseover method, in which you mouseover the actual models of the players clumped up and cast blossoms on whoever is below your mouse, this works well for blossom because you don't really care who specifically you target the spell on, just that this person has injured allies around them.

As a final note on this, do not cast blossom on random injured ranged players just because you see them drop to low hp in your raid frames, the spell does not work that way and you're doing more harm than good.