Not sure if Slamm scooters are threaded but probably.
Under the clamp will be two bolt thingies which need to be tightened with a wrench. (that's if it's threaded)
If it is threaded... then you need to tighten it with a wrench or spanner(s), to tighten my threaded MGP headset I personally use these: http://skatehut.co.uk/acatalog/Headset_Wrench_4_in_1_.html
Yes they are threaded. Just find a wrench that fits the big nut below the clamp. Tighten the bottom one, then the one on top of it, then make sure your clamp is tight.
Tighten the headset and clamp. Threaded headsets dont spin very well when tightened. At least not in all of the expereince ive had with threaded headsets.