Username of the sockmaster or suspected sockmaster
Just include the name, no "User:" or square brackets.
Second positional parameter
spi and suspected - these are equivalent, and can be used if the user has not yet been blocked or confirmed by checkuser.
blocked - this is used for sockpuppets that have been blocked on behavioural evidence (the duck test).
proven and confirmed - these are used when the sockpuppet has been confirmed by checkuser.
nbconfirmed - this is also for when the sockpuppet has been confirmed by checkuser, but mentions the Checkuser specifically ("A Checkuser has shown that this account is a sockpuppet of X" versus "This account is a sockpuppet of X").
If no second parameter is specified, the template outputs a generic message: "An editor has expressed a concern that this account may be a sockpuppet of X".
Third positional parameter
Evidence of sockpuppetry
This field can be used to link to the evidence of sockpuppetry, or to give a very brief summary (a couple of words). If this parameter is left blank and the sockpuppet investigation case page is a blue link, then a link to the case page is provided automatically.
Length of time the sockpuppet or suspected sockpuppet was blocked
Use this field if the sockpuppet or suspected sockpuppet was blocked for a specific length of time. This parameter defaults to "indefinitely".
Custom category name for the sockpuppet or suspected sockpuppet
This parameter defaults to the category "Wikipedia sockpuppets of X" if the second positional parameter is set to "confirmed", and "Suspected Wikipedia sockpuppets of X" if the second positional parameter is set to anything else. Here, "X" is the username specified in the first positional parameter.
This template adds pages to one of (as appropriate):