Set your generator for 120/240.
Get short section 4-conductor #8 copper cable, an 8'x1/2" ground rod, a ground rod clamp, a 50 amp receptacle that matches your plug, a box with cover to house that receptacle, and a cable connector for you cable (to attach the cable to the box).
Make up the receptacle according to the markings on the receptacle.
L1 or X = black conductor - L2 or Y = red conductor - L0 or W = white conductor G = green conductor
Mount the box on a pedestal or on the side of generator where convenient.
Install the receptacle in the box and install the cover.
At the generator use cable ties or some other method to attach the cable to the generator (strain relief for the terminations), terminate the black conductor on L1, terminate the red conductor on L3, terminate the white conductor on L0 and terminate the green conductor on the ground lug on the frame.
Drive a ground rod 8'x1/2" or bigger and run a #8 copper conductor from the ground rod to the ground lug on the frame. The ideal situation would be to use a longer length of that 4-conductor cable so that you can leave the green wire long enough to run unbroken from the outlet, through the ground lug on the fame and finally to the ground rod. Trim the excess black, red and white conductors in that cable to keep things neat.
Receptacle-----------Generator
L1-(X)------------------L1
L2-(Y)------------------L3
L0-(W)-----------------L0
G------------------------GND_LUG--------GND_ROD
http://www.dasplace.net/RVWiring/125250v3p4wgrnd.jpg
http://www.dasplace.net/RVWiring/DSCN3897-220SocketBack.jpg
Most likely you will need this type of receptacle.
http://www.dasplace.net/RVWiring/DSCN3900-220Socket.jpg
http://www.dasplace.net/RVWiring/14-50.gif
If you're still not sure call a local electrician.