Branding is not actually abusive if agreed to by both parties. It can constitute abuse, however, if not done correctly and disfigure the subject.
Most people are unable to withstand the brand, They either need to be held stiffly or restrained adequately. The subject should NOT be able to move, twist or jump. If they do, there is a significant risk of ruining the brand, disfiguring or causing serious injury to the subject. A competent approach is to restrain the individual at the hips, right at the start of both legs, the middle back, and just below the shoulders, as well as above the knees. The restraints should be tight. This will hold the individual securely while the branding takes place.
The Branding Iron
Many companies sell branding supplies for cattle and livestock. Their standard irons frequently are too large, in my opinion. Many of these firms make custom irons for a slightly higher fee, so getting one made smaller should be no problem. Prepare the unit per the instructions that come with the branding iron.
It should contain no circles, if possible to avoid the cookie cutter effect, which when improperly applied lifts patches of skin from the subject's body as the iron retracts from the body. Similarly, avoid fancy loops as well, same reason. Most use an initial, or two, or numbers.
Preparing the Brand
The brand should be as hot as possible. Otherwise, you will have to keep it there longer than either of you would like, and cause undue pain to the individual. The brand will need to be held on the spot with firm pressure for at least three seconds to be permanent. Make sure the branding iron is adequately cleaned before use, and afterwards.
The subject will complain of the intense pain. Therefore it is prudent to have the person gagged, or provided a bite chomp. They can hold this tightly between the teeth so the biting response will muffle cries of agony. The individual will scream as the branding occurs. This is normal. There will be a lot of pain during, and afterwards.
The mark of the brand needs caring for properly. I would suggest icing the spot with a simple cube. The cold sensation will quickly dissipate remaining head, causing the swelling to not be as exaggerated. It also provides soothing relief to the nerves, and naturally washes the burn of any dirt. Proper care will avoid unwanted damage the mark.
Pain pills would be handy if you prefer the slave not suffer overly much. A good ointment can be applied later, like hydrocortisone cream, or salve. Once the risk of separation elapses (usually 20 minutes), the area should receive burn treatment salve and kept covered until it heals properly. The bandages need frequent changing (twice a day minimum), and the mark needs regular checking for infection.
At the first sign of infection, antibiotics need to be administered. If these guidelines are followed, you will have a properly branded the subject. I hope this helps.