The site
A micronation needs to have a defined territory. For instance, many micronations create artificial islands as living space:
You can also claim territory on the seafloor or in outer space. The main advantage of these is that you can claim these for purposes of research.
Flag & Coat of Arms
Independence
See Independence
Political and legal decisions
Constitutional state
the third requirement for states is a government, to be recognized by other states micronations need to have a constitution defining and/or describing their government structure and function.
Establishing sovereignty
A micronation must establish sovereignty and the name of that micronation known to other states. To make their sovereignty official they draft a declaration proclaiming how and why they created a sovereign state including legal reasons why their sovereignty is valid and send it out to surrounding states.
National Defense
Defending territory against outside forces (pirates, for example) is a tough problem to overcome. If you have made treaties with other, more powerful nations that require them to aid you if you so ask of them, this may be of great benefit in terms of defense. However, in the case of a small man-made island, the outcome of an attack against the island depends greatly on whether the other nation's forces arrive too aid on time or not.
a state may happen to have the resources to build it's own defense force. a state may set up signs around their island to warn outsiders of their existence and their right to defend themselves. Creating a defense force requires, tactical and strategic placement of military resources. most micronations don't have the money or ability to own and/or maintain large scale military firearms and thus they must rely on more basic methods. Concrete road barriers and entrenched infantry positions are useful for defending against relatively short-range attacks from the sea, this does require an island with enough room to store ammunition and hold up to recoil, as well as some way to repair or evacuate the island in the case of an emergency. However, even if a state has the firepower, the state must allocate substantial human resources to defend it's territory which in the context of a micronation is somewhat difficult.
Planting mines *may* be against international law; a state must declare that it has mined an area, but how specific it is in declaring it is completely up to them.