Off camera flash is kind of a hit and miss thing but i can give you some pointers to get you close. First off try to position the flash so you are only lighting your subject and not the bushed and ground like in the photo above.
Use a narrow beam. I usually will shoot at 50mm to 70mm zoom on the flash and point it so it is only lighting my subject. I'm not sure if your flash even has this function or not.
Here are a few examples from Sat.
This was shot with 580 rider right and one in the shoe. I like to shoot cross flash. My shoe mounted was set at a very low power and wide zoom and my off camera was 1/8 to 1/4 power with a medium zoom. This was shot very close with a 15mm lens.
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This was was with the 70-200 2.8 lens and flash rider left and probably 20 ft in front of him. Not liking the shadows from the handlebars but just posting some different angles.
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This was rider right and fired when the rider was about 10 ft from it. I was down the trail a bit shooting at 200mm. Again low pointing up with a narrow beam around 1/16 power because I'm shooting at 2.8
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This was similar to the first. Off camera is rider left and on shoe also with 15mm lens.
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You will need to do some trial an error but is not too difficult. Let me know if that helped.
