Nate, when do you want to cook, right away, or tomorrow?
if you do have time, it goes well to marinate those.....not absolutely essential, but it does add to the finished product....
If you're going right away, maybe just salt, pepper & possibly Worcestershire sauce for a short soak.....
fairly hot fire, on one side of the kettle (40+ coals?) so you have a cooler area to move it off to....
I do sear 1st, then move to the cooler side & "bake" for a short time 'till doneness is what we like....
A lot of folks do the reverse of this, cooler side 1st, up to 10f under the 'done temp & then over the hot coals for just a few minutes per side.....not very long at all....
For flank, most folks go to med. rare, if you have a way to check internal meat temp, that will be about 135f or a bit less in the center of the piece.............
it's hard to guarantee you exactly "how long", as the piece size, the kettle temp, which sear you do, 1st or last will have various effects.....
as a GUESS, for a standard one, maybe a foot or so long & 1-1/2" thick, 25 or a bit more minutes on the cooler side plus searing.....
Let it rest, loosely tented w/ foil for 15min. before you cut, then cut across the grain & at an angle (on a bias) & it should melt inna mouth !!!
I hope this gives you enough to go on......
Gotta go, but others will be around for ?'s !!!!