In my experience in removing my factory S1 front bar, there were 2 bolts either side where the front bar meet with the guard, near the wheel arch.
There were then 4 bolts underneath the front bar, staring the the ground. A simple philips screwdriver will get these 4 out. There are 2 on both sides.
There were then 3 clips underneath the grill. When I say grill, I mean the little section that clips onto the headlights and sits between the bonnet and front bar.
Sometimes, you will get lucky and removing these 8 bolts and 3 clips is enough. However, there is a large clip underneath each headlight that the bumper slides under. A flat head screw driver to pry these up will do the job. If you haven't got a tool suitable, you may need to remove the headlights. If this doesn't help, send me a PM and I'll ATTEMPT to draw up a diagram.