Hi there and welcome:)
This is a great time to get started as they are just out of the 'fourth trimester'. I'm posting a few links that you can go through. There's a ton of information/ advice and helpful tips from fellow members. To get started I would say just follow a rough routine. The essence of BW is following your baby's cues. The most important thing is to simply stay consistent.
Starting EASY - all you need to know and more!
https://babywhispererforums.com/index.php?topic=186622.0Sample EASY Routines from 0 - 13mths+
https://babywhispererforums.com/index.php?topic=164253.0Is it to confusing to have some naps in the sling still while ST? We wan't to be able to take her out when doing things whith her older sister.
I would suggest for a few days, while you are getting started, stay in and stick to your routine. It's just so that you can be consistent. After she gets used to it, you can adjust as per your family's needs. A sling is super handy to have. You can always plan for one or two naps to just be in the sling and work that into your routine. With my daughter, we did all the long naps in the cot but the evening catnap was in the sling.
If she doesn't go to sleep in the cot do we give up after a certain amount of time and do the sling to protect her getting OT or will that ruin everything?
Yes, absolutely use the sling whenever you need to
I also cosleep with her at night which I was going to keep doing unless it's also to confusing for her? I'm thinking that her day and night sleep have been diffrent already but similar in that she has bodycontact..
I have never co-slept with my kids so no BTDT experience but it could be confusing. If you are fine with it, you could put her to sleep in her cot at night also. It is safer for them. It is recommended that baby sleeps in their parents room for at least 6 months
The last thing I'm thinking about is our wind down, the times I've tried with the cot I've walked around with her a few minutes in the darkened bedroom with white noise like we do when she's in the sling but is that enough? I've tried just sitting with her before but she just wriggles.
This should help What does a good wind down consist of (Includes 4S ritual)
https://babywhispererforums.com/index.php?topic=212645.0All the best! We're here to help you through this. What I LOVE about Babywhisperer is that it helps us to connect to our babies and understand them better. There might be tears but you will always be with her guiding her through it. Let me know how it goes. Xx