You need to find the quick which is the blood supply in the nail. You will want to trim the nail avoiding trimming through the quick. If you do it will hurt and bleed.
If your bun has dark nails holding a flash light behind the nail will help you to see the quick. You may need someone to help you if your are using a flashlight since holding a rabbit, the nail clippers and a flashlight could be difficult. If you can't get someone to help you, you could try propping the flash light up on some books or some other object.
You can use cat nail trimmers or bird trimmers for your bun. Try not to cut through any hair as buns need the hair to cushion their feet since they do not have pads such as dogs. Also, if the clippers pull the hair your bun may jerk while you are clipping.
Once you see the quick you can then place the trimmers on the nail and give a firm squeeze at first without cutting through the nail. Pay close attention to your bun. If at this point your bun jerks it's foot or squirms you may be cutting too close to the quick.
If your bun is not used to having nail trims and seems really stressed by the process then you could try just doing one foot a day until all are done. If your buns nails are very long then cut off some of the nail. A week later cut off a little more nail. The quick grows as the nail grows so when they are very long you can cut a little off once a week and as the nail gets shorter the quick will recede getting shorter.