First, obviously, play on Casual difficulty.
Without glitching, do metal or herb Requisitions for Power, until Inquisition Influence is level 6, then do the War Table mission to get Power for a Price. Then you can turn gold into Power and Influence. You can do Astrariums for pure Influence, no XP. War Table missions are also good for pure Influence.
You could do other types of requisitions, but sometimes the enemies don't drop the items you want and it's a bore trying to get all of them (I'm talking about you, Malachite!). Metal and herb are deterministic, you know that you can get everything you need by just working at it.
The speedruns also lay out a minimal path for side quests that achieve just the right number of Power points to get to Skyhold. You can get 18 Power in the Hinterlands with minimal effort. That's all you need to get to Skyhold and Power for a Price. Roughly, it goes as follows:
- Crossroads
- Upper Lake camp
- Templar HQ - take out the Templar boss, completes the quest given to you by Mother G
- Farm camp
- Horsemaster and get a horse
- Close the Farm Rift (immediately adjacent to Horsemaster)
- (Return to Haven to open Val Royeaux -- do all that and pick up Sera and Vivienne and Belle as an agent -- then come back to Hinterlands)
- FT back to first camp, then ride to the rift by the Agrarian farmhouse, close it
- Ride to Dwarfson Pass camp, pick up missing scout side quest on the way - for the agent, and the phylactery for another agent
- Complete the missing scout quest (MAKE SURE YOU HAVE VARRIC WITH YOU!) and pick up the Scout as an agent
- Ride to Winterwatch tower and do all the side quests in there, including closing the rift and getting Speaker Anais and Lord Berand as agents
- Close one more rift, the southernmost one in Dwarfson's Pass is handy, though a bit high level
- Pick up Enchanter Ellandra as an agent in the Crossroads (MAKE SURE YOU HAVE VIVIENNE WITH YOU)
- Ride to Redcliffe Village and finish all the main quest steps there
At this point you should have 18 power. If you are short a few points, close a couple more rifts or do some requisitions. Then for the rest of the game you don't have to worry about getting power -- assuming you don't open any new zones that aren't absolutely necessary for the main quest. If the main quests or incidental side quests on the way don't give it to you, requisitions or Power for a Price or closing more rifts will.
Don't worry about XP, you don't really need it. You can beat the game at level 12 on Nightmare. But if you really, really want XP fast, fight dragons. I went from Level 18 to Level 20 by fighting two of the dragons in EdL. Short of that, you can take the Perk that gives you a 5% increase in XP for killing fools.
Now all that said, some glitches were fixed, but new ones were introduced. I think every glitch in the speedruns now has an alternative glitch that is as good if not better. Though I haven't tried it myself, it should be possible to get the Amulet of Power from Crestwood and dupe it to get as many skill points as you want.