I want to insert variable data into an Outlook template

Start with opening the VBA editor (press alt+f11). Paste this example for how to use the NewInspector event into the ThisOutlookSession module. You must edit the example else it would change the subjects of all of your items that you'll be opening! I don't know how to identifiy your template, if your template can be identified by its subject, the code could look like this:

Private Sub m_Inspector_Activate()
  dim Item as MailItem
  dim Value as string
  if typeof m_Inspector.currentitem is mailitem then
    set mail=m_Inspector.currentitem
    if mail.subject="your template's subject" then
      value=inputbox("enter the first value")
      'this replaces '#0#' by the entered value
      mail.body=replace(mail.body,"#0#",value)
      'repeat the above two lines of code for every value.
      'don't forget to change the name of the next placeholder as well
    endif
  endif 
end sub