Consult your veterinarian. The selection of needle size may depend on the product you are using. Provide a clean table for a workspace, to place syringes, needles, medicine, etc. Choose a skin site for injection that is clean, to avoid carrying bacteria into the injection site. Follow the directions for the product you are using - read the label or ask your veterinarian.
Some products have specific directions or warnings.
Avoid injecting an excessive volume of product at any one injection site - usually the product label will address this. Establish a good herd health program, including appropriate vaccinations. This will help prevent illness and losses later. Work with your veterinarian to evaluate your program and make changes as needed. Because I sell my calves as feeders, I do not need to worry about injection site lesions since most of them result from feedlot vaccines and treatments.