If you can wire up a household plug you can fit a DIY motorcycle alarm. Just select one you can find online. Most not cost that much and they are really very simple to install. Some aftermarket motorcycle alarms are approved by European insurance companies and will cost a lot more and we know not everybody can afford that. Anyway, if you going to install your own aftermarket motorcycle alarm, you should know a few things.
Positioning the alarm unit is the first job. For a quick install you can use one of the rubber bands that hold the tool kit or a lock in place under the seat. For a better job use cable ties, small bolts, Velcro or the sticky pads from number plates and stash it out of the way somewhere. It doesn't have to go under your seat but that's the most obvious place.
To get the alarm working, connect the red wire to the battery positive and the black to anywhere on the frame which will act as an earth. That's it – there's no need to do anything more if you don't want to. For a more secure installation use a live feed rather than the battery positive. If you'd like the indicators to flash on arming/disarming and when the alarm goes off, splice the alarm's yellow wires into them. Generally this is easiest at the rear of the motorcycle. Use a multimeter (or a bulb) to identify which wire going to each indicator is the positive and splice into that, ideally by soldering and using quality tape or heat shrink.
If you want the LED which shows the motorcycle's alarmed, it's easy to fit. Choose a suitable location – matt plastic are easier to drill than gloss ones. Cover the area with masking tape and drill a 5 or 6mm hole (start the 5mm). The LED should press fit and connects to the main unit with spade connectors.