I am a bit short of time at this moment, but can I ask first what level you are at the moment in glass repair, this will help me to decide which way to explain it.
I must add that although I like long crack repairs the majority of people making a good living from WSR do not do them.
To save a bit of time I have posted a write up from Brent from a while back.
I have done so many times on this forum and elsewhere, so I would ask that if you want a real discussion on the matter that you give me a call. I will however give you the abbreviated version.
If the crack is over 3" long and radiates from an impact point I first warm or cool the glass as necessary and dry out the crack, then drill the end and pop a bullseye, making sure to blow out any dust or loose glass from the drill hole. I then set up my bridge at the impact point and start injecting resin just as I would with any other repair. There are a number of factors that determine how far the resin will travel from the impact point but in some cases it can travel 6" or more without touching a thing. I then cover the crack nearly up to where the resin stops, but leaving at least 1/8" of the filled crack extending beyond the curing film. Where the resin stops I use an eye dropper to place a tiny drop of resin just "behind" the point at which the resin stopped (not over the unfilled crack). How much resin is dispensed from the dropper is easily controlled by placing the tip of the dropper against the glass as the resin is applied. From the point where the resin is applied at the surface it is wicked into the crack. With every drop the crack will fill from .25in to 1" or more. As the crack fills I cover it with curing film a few inches at a time. When the resin reaches the end of the crack it will sometimes fill the mini bullseye that I created without any further interaction. If it does not fill I place a straight pin in the hols and run a little resin down the pin to fill the bullseye and drill hole. If there is any air still remaining I will attach a bridge at this point and inject resin into the mini bullseye, but I prefer not to if I can help it. Cure, scrap, polish, done.
Why don't I slide a bridge? In my experience sliding a bridge applies pressure and partially closes the crack. As that pressure is released the crack opens back up and sucks in surface air. Using this method you can fill a crack from side to side, top to bottom and bottom to top. Most people are surprised when I tell them I prefer to fill a crack from the bottom up rather than the top down as it is easier to control the flow of resin.
Why don't I use an injector on each end? As puka pau said, that often leave a bit of air trapped at the point where the resin meets in the middle. Also, some cracks are prone to run even after being drilled and popped so I'd rather not put any extra pressure on that volatile point.
Why don't I use crack expanders (crack jacks)? Normally they are not necessary to fill the break, but I will use them if I have a particularly stubborn crack. If I do have to use one I put it on and leave it on until I have the crack filled and covered with curing film, removing it just before I'm ready to cure.
Why don't I flex the glass as I'm filling? This often introduces air bubbles into the crack that are very difficult to remove.
Filling from one end to the other without any other manipulation pushes the air out of the crack and provides the best results for me. Some think this is nothing more than "dribbling" resin across the surface of the crack, but in reality I am using the pressure from the injector to displace the air in the crack with resin. If I don't get in a hurry and get ahead of where the resin stops as I apply additional resin to the surface there are no real limitations to the length of crack I can fill. (That does not mean that I recommend repairing a crack that runs from one end of the windshield to the other.)
Over the years I've had many, many technicians tell me that after practicing this method a bit they get better results than with their previous methods, but only you can decide which method works best for you and which method that is may be in part determined by the injector and resin used. I also only use one viscosity of resin where some technicians prefer to use several. People sometimes think their way is the only way, but I have an endless supply of glass to practice on so after 24 years of repairing glass I find I'm still learning all the time. To each his own as long as you get the crack properly filled and it does not crack out after it is cured.