Firstly as Luddite has helpfully listed I would check everything (front and rear) is set to standard before you go fiddling- it's not unknown for bikes to be delivered with settings that differ from those suggested by the factory let alone what a previous owner (if there was one?) has twiddled with.
Next would be to set the 'Sag' which you will need a friend(s), some masking tape, cable ties a tape measure and pen and paper.
Unfortunately, the chances are with this measurement that it will likely be a compromise as the only way to get this right (unless you are very lucky) with you sat on the bike will be by changing the spring weights.
Getting the above somewhere close will require you to adjust the pre-load settings but doing so alone will not compensate for not having the correct spring based on your weight hence my comment on it being a compromise if you do not get this spot on.
However, they will form the basis of any further adjustments and without these being right first, nothing else will be really.
Here's a good explanation of how and why:
https://racetech.com/articles/SuspensionAndSprings.htm
That's the tough bit done- now it is the 'ride it and twiddle test' until you get the kind of 'feel' you are looking for.
My advice would be a circular route that you know well but not too long (say five or six miles tops) that has a combination of road surfaces, bumps, fast and slow corners and basically ride how you do normally- do a loop, come home and make notes of how it felt with all the settings (except the already adjusted sag settings) on standard.
Now you can start softening things if it felt too hard or stiffening it up if it was on the soft side- there's not really a wrong or right here, just what feels best to you.
Obviously, there will always be more that can be improved with spring and oil weights, shimming and ride height/geometry etc. but it really depends on how far you want to go and how deep your pockets are along with how much time you want to spend riding Vs fiddling.
Once the sag is set, depending on how much you can notice with each adjustment (I doubt many 'normal' riders on the road would notice a difference of a couple of clicks at a time) I would say get out while it's all fresh in you mind and get it set in a day max.
I would also say that properly set up suspension is one of the most underrated and last thought about 'mods' but (in my opinion) is one that makes every ride so much enjoyable no matter whether you're pootling about town or going nuts down a country lane because you can 'feel' the difference it makes.
Have fun
