The outside part of the chainring bolt usually has a hexagonal socket which takes a hexagonal key. The inside may take a special key with two teeth or a hexagonal key. If you don't have the key you might be able to do the job with a big screwdriver, but take care. Sometimes the inner ring of a three ring system is bolted directly into holes on the crank spider. These bolts may have a star-shaped Torx socket that needs a specific tool.
Grease the Components
Grease the threads of the bolts. This allows them to be screwed tighter with less force. Keep grease off the outside of the 'female' elements of the chainring bolts. It's easiest if they tend to stick rather than rotate.
Position the Components
Some chainrings have a stud to stop the chain jamming between them and the crank. position the chainring so the stud is next to the crank. If the chainring is not symmetrical - sometimes the teeth are slightly off-set to the left or the right - put it the correct way round. If you don't know which is best you may need to mount it and check the operation of the front derailer.
Work in Stages
Connect all the fixings loosely in position first. Tighten the bolts in stages working across - not around - the circle. This allows the chainring to settle in the right position and spread the load evenly between all the bolts.