When and how to start independence in Colonization?

I didn't understood when and eventually how can I start independence in Colonization. Can you explain?

From the Civ Fanatics Colonization FAQ:

What are the requirements for declaring independence?

The only requirement for starting a revolution and proclaiming independence is reaching a level of 50% for "Rebel Sentiment". Rebel Sentiment measures production of Liberty Bells, somewhat like Culture in Civ4, across the empire as a whole.

At this point the European King sends the 'Royal Expeditionary Force' to stop you. You now have to fight a war of independence. As such you want to be in a position where you have enough resources (guns, horses etc.) to fight a prolonged war.

Again from the Civ Fanatics Colonization FAQ:

How can I possibly defeat the King's forces?

...you cannot build stronger units than the European King. Thus, you must be patient and plan for the long-term: since the REF comes in waves, make sure to take advantage of lulls in fighting to replenish your resource stocks. Also, do not waste too many units trying to go on the offensive -- often, it is more efficient to engage in a war of attrition, using the settlements' defenses and terrain defensive bonuses to wear down the REF. The timing of your declaration of independence is also important. Be certain that when you declare your revolution, you have enough key resources, like Tools, Guns, and Horses, to convert colonists to soldiers in case of an emergency. Having a homegrown Gun industry would also be beneficial and reduce dependence on Europe.