I started getting my bengals (i got 2 at the same time at 6,5 and 7 months old from the shelter) used to the harnas after a week, just indoors. And started going outside after 2 weeks.
I live in a building which used to be part of an hotelcomplex (the house i live in has been renovated and there are now 4 appartments in, i have my appartment upstairs), so i started with them walking on the parkinglot which is empty or almost empty during the day. At the start they were very afraid of cars as well, and needed to built confidence in the surroundings and me before i would go on to the sidewalk.
Then i went from one parkinglot to the other (at the backside of the buildings) and let them get confidence there, from there up to the sidewalk, walk the sidewalk for 50 m till my frontdoor, and when confidence was there (with the cats and me) i started going to the sidewalk directly.
I only have to walk the sidewalk (with slowdriving cars on the road btw, i live in the centre of an old village, not built for fast driving) for about 50-100 m and then turn into a smaller road with hardly any cars driving by... and on to another (even smaller) road with even less traffic, from there im out of the centre and its another 200 m (a few small streets with no traffic during the day) to the forest.
With Ayla i have been in the forest for about 2 weeks now, Finnley isnt there yet, we are up to the 'last' street before the forest.
I talk a lot to them, praise them, go through my knees, pet them during our walks. If they are insecure i pet them at their bottom/tailarea, once the tail is up again their confidence is back... at least for that moment.
Sometimes i carry them for one street (Ayla) or just a few meters (Finnley, he wriggles).
If they come across something scary i let them look and after a while 'ask' them if they wanna go on again, Ayla is easily distracted with a wandtoy i also carry with me... to get her moving again, ofcourse i dont use it on the streets with cars (passing by) but she also freezes up a bit when we see other people/cyclists, or wants to go into gardens of houses/people i dont know.
If we come across a dog i pick them up. They are a bit impressed by dogs, if we encounter them i pick them up. I try to avoid comming close to people with dogs (cross the street to the other side if i see them for example). They are actually not too afraid of dogs once they get used to them, but i dont know how the dogs react to cats or a cat on a leash.
As i said Ayla picks it up faster then Finnley. He was a bit more insecure at the start but picking it up fine though.
I cant really distract him with a toy when we are outside, he isnt very interested and wriggles more when i pick him up. However i can see in his behaviour precisly when he has had enough 'stimulation' and starts to get taking in 'too much' (which i like to avoid ofcourse), sometimes we dont get further then the parkinglot, sometimes we get almost to the forest.
However my cats are a bit older then yours is.
Keep in mind that they can bolt on you/the leash when they get too afraid/overstimulated, so make sure the harnass fits perfectly and cant escape from it. Take babysteps if your cat asks it from you.
Read her behaviour and bodylanguage, what does she like?
Finnley is (was) more the sniffing each and every meter from the parkinglot, while Ayla was very interested in the sound of the birds and the wind blowing the leaves (bless for autumn), i started both to sniff trees (by using the toywand), standing up against them etc. Since marking is natural behaviour they love that.
If your cat gets scared, reassure her as much as you can, once the 'danger' has passed (dog, car, strange noise) distract her if possible . I experienced that Ayla started asking for reassurance when she got a bit insecure.
I have both my cats on a short leash, i think 1.5meter, will get a longer one in future but for now i like to be very close untill both me and the cats are very much used to it.
Each cat is different, take your time and dont overhaste things.