Marrying the soft flank feathers is the same as marrying goose/turkey etc.
The key difference for me is keeping the fibers attached to the stem while you marry and tie them in.
One mistake a lot of folks make is using fibers which are too big - i.e when it comes to tie in, they are not tying in on the optimal portion of the feathers. (this goes for all feathers, not just the flanks in this portion of the fly)
You need to be tying in on the softer root area, which can usually be found on the bottom 1/ 3 of the fibers, closest to the stem.
Keeping the fibers on the stem at tie in also helps with positioning and retaining the marriage.
Your fly looks good, and if i may make a few suggestions for your next attempt
1. make the butt about half the size of the one on this fly.
2. use a larger tinsel for the rib, this looks to be the same size (perhaps smaller) than the tinsel on the tip. Typically i use med tinsel for ribs, depending on the size of the fly/hook - this hook could certainly take med tinsel.
3. Try to keeps the sides - in this case, woodie and jc, above the plane of the hook shank.
4. try to keep the hackle more swept back, rather than down like they are here. Fold the hackles, and use 2-3 turns of the blue, and 1-2 turns of the guinea
5. head could be much shorter, the red wool part is well done, the black varnished part should only be the curved portion.
I understand you said you need to work on heads
Take this info as you see fit, and just so you know i'm not talking through my hat, here's a silver doctor i tied recently (from Fisher - there are many different versions in the classic salmon texts.) .