Plan out what alters you'd like to have, or at least the ones you'd like to start with. I find it easiest to use Pinterest and make Pinterest boards of alters. Plan out their identities, anything you'd consider important to their personality and individuality.
Have a look at manifesting tips from willogenic and tulpa systems. Trying these methods will result in your alters! It might take a few methods to find the right one for you.
Think about what types of amnesia you'd have. Will it vary from alter to alter, or will you consistently have one type. We have mixed amnesia types, and what each alter has access to is dependent on their role and if they're on good terms with our gatekeeper.
Download a tracking app (rip simply plural). I would recommend pluralspace if you want to be discrete because it's available on a web browser not only as an app. You can start mapping out and adding alters before you create them as a type of visual aid.
Read up on DID. There are lots of educational resources out there, and educational accounts on social media that discuss dissociative identity disorder.
On the same note as the previous, research the different types of DID. Partial-DID, Polyfragmented-DID, Complex-DID, Highly Complex-DID, etc.. if you have a need for one of these your research will have to be more in depth. This would require thinking about possible subsystems, sidesystems, and other more complex structures within your system.
Consider your innerworld/headspace if you'll have any. You can draw or write it out yourself, but pluralspace also has some sort of mapping feature (which I have not tried, please do not take my word for how good it is).
Fake it till you make it. Pretend to be your alters, pretend to have alters. All of it helps.
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