Hi, busy,
The only kind of flint wall I've built is for a house, with flint panels in the front elevation, if you're repairing existing, or building a perimeter or retaining wall then the method will be slightly different.
however this is how I did it.
The wall needs to be built first, in 4"brick, so that there is an empty panel 4" deep with ties of some sort to hold the facing flint, I used strips of 6" wide expamet (expanded metal, comes in rolls and is used laid in the bed joints to strengthen a wall, but in this case it is cut into strips 18" long and left with 3" protruding to grip the flint).
So you end up with a 4" common brickwork wall with 9" brickwork at the corners, built with a block bond form, (in and out every 3 course) which stands 4" proud of the backing panel.
These corners then have the lines put on as a guide to keep the flint of fair face appearance (flat) although you do not try to make the flint run in courses like brickwork, it is totally random in pattern.
As you start bedding in the flint, you will need to trim it as required, (with a brick hammer) as you go, mixing large and smaller pieces, but trying to keep a 10mm joint in between as far as you can, after you've laid a couple of meters leave it so that the mortar can take it's first set and then with a pointing trowel when the mortar is semi dry, you can point up using the point of the trowel to cut a protruding 'V' shape into the joints.(bottom of the 'V' pointing out )
Brush off the next morning with a stiff wall brush.
I suggest you go and look at some examples.