I would agree with some of the posts mentioned above, if your heart is set on playing PLD and you feel like you're being forced into switching. Of course, this can also be viewed an opportunity for you to learn DRK (which also happens to be the most entertaining job in the game), if you have the interest and drive. I ended up relearning PLD last tier after neglecting it for a while, during A7S, and it was great fun. If you're motivated enough you can pick these things up extremely quickly, when the need arises.

The best way to learn a new job is to sit down and figure out what the priority system is. There's no point in copying a "rotation" without understanding what the first principles are, especially when the job doesn't really have a rotation to begin with. DRK is less structured than PLD or WAR, and you'll have to make decisions on the fly. Most of the different openers follow naturally once you understand the underlying reasoning. Let's look at single target for an example.

GCD Abilities:
Scourge: 500
DASE Combo: 267/GCD
Delirium Combo: 227/GCD

Scourge is your most important GCD ability, and maximising its uptime is essential. If you refresh it too late, you lose DoT ticks at 40 potency each. If you consistently refresh it too soon, however, you lose GCDs that could have gone towards an additional Delirium combo. The catch is, however, that it interrupts your combo, so you have to make a decision at the end of every combo as to whether to refresh Scourge or try to fit in an additional combo first. In addition, you play at two GCD speeds: one with BW, and one without.

Check what your GCD length is on the tooltip of any weaponskill. Also, activate BW and then check what your GCD length changes to. This should let you estimate how many seconds it takes you to complete a combo both with and without BW active. Try a few practice runs where you just focus on reapplying Scourge as precisely as possible. One more point about Scourge: it has to be active for a minimum of 15 seconds for it to be unequivocally better than any hit of your Delirium combo. So if there's less than 15 seconds on the clock, or if you expect to complete the target dummy within the next 15 seconds, you can let it fall off rather than reapplying it.

The choice for DASE vs Delirium is fairly simple. If you are in danger of capping MP, burn off the excess in DASE. Otherwise, feel free to use Delirium. The MP can be expended at any time; there's no rush. Someone once compared DASE to a pressure-release valve. I think that's a fairly accurate description. Of course, as you approach the end of the dummy, feel free to burn off your excess MP. MP management is fairly simple once you are aware of this, especially in single target. It's probably the easiest part of the job.

oGCD Abilities:
Salted Earth: 525
DAC+S: 450
Plunge: 200
DP: 150
Low Blow: 100
Mercy Stroke: 200

I've left out reprisal for obvious reasons (i.e. target dummy). I'm also not going to go into DADP here, because I we're talking about single target, and the priorities are different with multiple targets up. It's worth noting that DADP costs one DA usage in exchange for 100 additional potency, which is less than the potency gain from DASE over Delirium (one DA for 120 potency). So you won't be using DADP in single target. DAC+S, however, gives you 350 additional potency in exchange for one DA usage, so you will be using it. Thankfully, most of the single target priorities are consistent with their actual potencies, even when you factor in their individual recasts (MS is the exception).

The only tricks here are that you have to fit in these oGCDs in as quickly as you can, in the correct order, but without delaying any GCD abilities to do so. Some can be double weaved easily, others cannot, especially with BW up. As you probably know, DA must be used on C+S every time it comes up, as soon as it comes up. You can activate DA earlier if you want in anticipation of either SE or C+S (it has a 10 second duration, and can even be activated pre-pull), but you must be careful not to accidentally apply DA to DP. Also, this is probably obvious, but BW must always be used when it comes up.

Main points:
1) Maximise Scourge uptime with as little DoT clipping as possible. Don't reapply if the mob is going to die/jump in the next 15 seconds.
2) Use BW on cooldown.
3) Use DAC+S on cooldown.
4) DASE is your MP bar's pressure release valve. Delirium is your go to combo.
5) Avoid clipping into your GCD when double weaving.

That's the basics. A lot of it comes down to being aware of what's coming off cooldown, and predicting how you're going to have to prioritise and slot them into the spaces between the next several GCDs. Practice until you can do it on autopilot. The target dummy should be fairly simple to do even without food or pots, at your gear level. If you're still stuck for some reason, see if you can record your attempt somehow and get some people to help troubleshoot it.