While each item will be different in terms of how the mounting bracket is attached to the bike, there are a few rules which seem to be consistent across all bicycle tail lights. First, you should mount the tail light just below the seat of the bicycle (although many bike lights give you the option to try out multiple areas- perfect if you have multiple lights). This is where you should be able to gain most visibility to other vehicles on the road.
Secondly, you should always ensure that nothing gets in the way of this light. Longer coats, bags and so on may cover the light and negate the entire point of having one- which is to let your presence be known on the road.
Finally, make sure that any new light is secure enough to handle lumps and bumps in the road without becoming loose and/or detaching from your bike. Feel free to give your bike a quick run-around in the daylight to reassure yourself that the light has been fitted correctly, if required.
In terms of fixing the light to your bike, some of the newer varieties require no tools to mount your new purchase- this is perfect for those who might have multiple bikes or simply aren’t great with tools. In these cases, you are usually provided with a latch and strap combination, in order to fit the light correctly.