The answer is, "dead easy if you know what you are doing, but it appears difficult if you don't".
I trust you will find the following easy to do, I have kept it as simple as possible, using the basic equipment and the least expensive and it will work with any camera that gives you manual control.
The set up.
1. Camera on a tripod with a basic trigger.
2. A flash on a stand in a cheap soft box fired by another trigger.
3. A flash meter with reflective and incident reading capability.
3. A subject.
List of my Gear.
1. Nikon D810
2. Nikon SB600 Flash with dome.
3. Tripod and light stand.
4. 2 x triggers. Yongnuo RF-603N under £25.00.
5. Light meter. Sekonic L-308B
6. Neewer portable soft box 60 x 60 cm. About £30.00.
1. Use the base ISO of your camera for best quality. mine is ISO 64
2. Set your meter to your base ISO and shutter speed 1/125 sec.
3. Camera to base ISO and manual setting Shutter speed 1/125 sec, one stop down from your max sync speed. You may need to refer to your handbook if you do not know.
4. Because the soft box has an inside diffusion panel and an outer one plus the flash has a diffusion dome on it, the flash needs to be set to full power and the flash zoom needs to match your lens. I am using a 85mm lens so my flash zoom was set to 85mm.
How to balance Flash to Ambient, step by step.
First let me say that you can do all this without a meter but that will require a lot of guess work, whereas using a meter will get you there instantly.
First take an ambient reading of the room or garden, in my case as it was very cold. I stayed inside near the window, I took a reading of the incident light coming through the window. This gave me a reading of ISO 64 Shutter speed of 1/125 sec. at f 5.6.
I chose 1/125 sec shutter speed as it is 1 stop down from my normal sync speed, because the last picture will be with a shutter speed of 1/250 sec my max normal sync speed.
I set the camera to these settings and took the first picture.
As you can see the garden is exposed correctly but the subject is not.
Next I took a reflected light reading off the subject, I took this off the side of the face facing the window. This reading was ISO 64 Shutter speed 1/125 sec. at f 2.8.I set the camera to these settings and took the next picture.
This time you can see that the ambient light is 2 stops overexposed but the side of the face nearest the window is exposed correctly, this is not what we are after.
So the next step is to set up the flash, I set the stand with the flash and soft box about 45 degrees up and the same to the side of the subject. This 45 - 45 is always a good placement to start with.
The starting point for distance from subject to soft box is usually the same distance as the softbox measures from corner to corner (the same as you would measure a TV) so I set the soft box approximately 3 foot away from the subject. I then set the meter to flash with my setting which to recap were ISO 64 Shutter speed 1/125 sec. and fired the flash with the meter held just under the subjects chin. The reading I got was f5.6 which is the same as the incident reading I got from the window, so both inside light (flash) and outside light (Ambient from the Sun) were the same. If they were not the same you could adjust the power of the flash if you needed to, or move the flash closer or further away depending on whether you needed more or less light reaching the subject.
I set the camera to these settings and took the picture.
You can now see that the subject and the garden are now both evenly lit. This is an OK shot but not exciting, that's because even lighting is not exciting, so the next thing to do is to reduce the ambient light so that the subject is brighter and thus more interesting.
I kept the ISO and the aperture the same but increased the shutter speed by 1 stop so my camera settings were ISO 64 shutter speed 1/250 sec at f 5.6.
I set the camera to these settings and took the final picture.
The final picture shows the ambient light darker by 1 stop and the flash which is just illuminating the face giving enough light to make it stand out from the background. The final picture is a bit more interesting than the previous balanced flash and ambient light picture.
This set up can be used outside to better effect and especially when you have a Sun set.
Remember one thing from this set up which is important and if you need further explanation just ask. It is that Shutter Speed controls your Ambient and your Aperture controls the Flash.
I trust this has helped if you follow the set up as closely as you can with your equipment then you will always get a successful outcome.