It's wonderful you want your little girl to go out into the fresh air. Just know that one walk means daily walks, sometimes twice and three times a day -- for the rest of your life!!!
The first thing is to get the right harness. Many people here recommend the walking jacket. You can order them sized to your kitty. They have fabric and secure with velcro. Some cats do not like wearing a "cape" around them. The H harness is a great harness that attaches around the neck and behind the front legs. The leash attaches to the back of the harness to eliminate any pressure on the neck. These are quite adjustable to fit your kitty. You should be able to place a finger underneath to make sure it is not too tight.
Now to get the harness on the kitty! Note that this can take a while to get the kitty used to the harness. You simply put the harness on the cat! One of three things will happen.
1. Your cat lays on the floor and acts paralyzed. This new contraption around them has rendered them helpless and unable to walk. See if you can entice her with treats or toys to get up and walk. Once she realizes she can walk wearing the harness, she should be fine.
2. Your cat will fight as if the devil has just been placed on her back. If this happens, try to distract with a toy or treat. If you cat continues to fight, take the harness off and try again later. You may have to repeat this over and over again. Have patience -- but keep trying. If after a few weeks, you still are unable to keep the harness on her for any length of time ... forget it and try again in another year! Some kitties just aren't going to wear a harness.
3. Your cat will walk around as if there is nothing around her body! This happened with my Raiden and I attached the leash and we were outside in a couple of minutes.
Note that you HAVE to get her used to the harness before you EVER take her outside!!! You do not want her fighting the harness and getting out of it. Some cats are houdinis and can get out of anything!!! So make sure she's comfortable and confident and even "walking" her around the house a few times after she is used to the harness. My Raiden was an outdoor kitty for 6 years, so he really wanted to get outside and the harness was the only way he was getting out there after I took possession of him.
Walking your cat can be a great experience for the most part. I carry a large towel with me in case I need to ward off approaching dogs or to have something to sit on while Raiden explores -- or he decides to rest on the grass (we do a lot of that in the heat of the summer). Just know that one walk will result in a cry for another walk. Raiden gets two walks a day -- if weather cuts one short, I may add one right before dark. Our walks last about an hour at a time, so you're giving up a large portion of your day just to follow your cat around who is sniffing bushes, walking around in circles, and stopping every five steps.
Good luck and let us know how it goes.