Summarize this article:

A step in harness is great for dogs that go on long walks or hikes. It is a very minimalist design, with a wraparound style that reduces pressure on your dog’s throat. These harnesses have a single buckle over the shoulder, making them easy to put on and take off fast. In most models, the lead is attached to D-rings for an even distribution of pressure on the harness.  A step in harness with a thinner chest pad is good for dogs who do not like added bulk in their chest area.  A step in harness with a wider chest pad is good for dogs who don’t mind extra bulk. The wider chest pad will give the harness more comfort and coverage. The wide size will also help to stabilize the harness in case your dog pulls on the leash during the walk. Look online for a step in harness, or at your local pet store. Most step in harnesses come in small, medium, and large, depending on the girth or belly measurement of your dog. Note that most harnesses are not adequate restraints in a motor vehicle. Do not use a regular harness as a seat belt /restraint unless it specifically says on the packaging that it is designed for this purpose. It should be open, with the top of the buckle facing the ground. Keep some treats close by to reward your pup once you put on the harness. Then, place her left leg in the left loop and her right leg in the right loop. You may need to pet her and coax her to stand over the harness. You can also enlist the help of a friend or partner to help you place your dog over the harness. Reach around your dog's neck and snap the buckles together over her neck. Step back and look at your dog in the harness. Does it look like it is pinching or squeezing her? Does it hang too low on her body? Use the adjustment points, or clips that are attached to the straps, to adjust the harness so it fits her properly. Most step-in harnesses have adjustment points on either side of the straps around your dog's neck and on the straps around her belly.  The harness should be snug but also loose enough for you to fit two fingers comfortably between your dog and the harness at any point. Test the fit of the harness before you attach the leash to the harness. Only give your pup a treat for a job well done once she is in the harness and ready for a walk.

Summary:
Purchase a step in harness. Unbuckle the harness and lay it flat on the ground. Have your dog stand over the harness. Secure the straps. Adjust the harness to fit your pup.