I will open my custom drawing template and create any symbols in there (Manage Tab --> Symbol) Then save. If you are in the middle of a drawing file, just create the symbol within your drawing file (Manage Tab --> Symbol) and then be sure to copy it from the current file over to the template. This way it will always be availble when you create a new idw. Also, it is good practice to save your symbols in another location for the odd chance your templates become corrupt. I created a drawing with all the symbols I have made and constantly update it when new symbols are available.