Buy some Quick Klot ( styptic powder)...usually sold where ever dog clippers are sold. If you do by chance cut a bit too much off and there is blood...just dip the end of the nail into the Quick Klot powder, the nail will stop bleeding.
Also, if you do make a nail bleed, don't make a fuss or feel sorry for the pup...dip nail in the styptic powder and keep on clipping.
The less of a fuss you make, the less of a big deal it will be for the pup. No dog ever died from having a nail cut too short.
Many Rottweilers do not like their feet touched, or their nails cut...if they start fussing when they are puppies...and you give up...they have learned that you have given in. Just take your time, even if you only get one paw clipped per day...it's best not to give up. Do it when they are tired and sleepy.
Gina