Warmth - Shelter - Water - Food
As soon as you recognize you are in a survival situation, you should assess your available equipment and start planning how you can maintain or gain warmth. Even on a warm summer evening this is the primary concern. If the weather suddenly changes and darkness falls, you may not be able to prepare a fire. And even when conditions are not adverse, a fire will provide you with a sense of security and comfort. As well, the light and smoke can attract attention.
Location - choose a location away from overhanging branches in the open where it can be seen from above. And clear a space free from twigs, grass, etc. (dig a pit, if possible). You don't want to risk a forest fire to contend with as well. You will also want to be thinking ahead to shelter, since location will again be a factor.
Preparation - gather a supply of tinder, such as dry twigs (break from trees rather than picking up possibly damp twigs on the ground) shredded birch bark, pine needles, etc. You may be carrying tinder in the form of cotton balls, steel wool or paper, or fire sticks. Have a good stock of dry wood available to add to the fire.
Sustaining - remember you will use an armful of wood an hour on average. The time to gather it is when there is still light. Once your fire is burning well, you can utilize some green wood or even slightly damp wood, if necessary. But, don't smother your fire. It isn't necessary to cut your wood, just lay a length across the fire and as it burns through, push the remaining pieces into the fire.