We need an XML file. You can either use the New-Item cmdlet to create one, or simply use your favourite text editor. We use Visual Studio Code for this recipe. 

1. Open your text editor and create a new empty file, and save it as CustomTypes.ps1xml.
2. Add the following content to the XML file. Ensure not to change the case.

<?xml version="1.0" encoding="utf-8" ?>
<Types>
 <Type>
   <Name>System.IO.FileInfo</Name>
   <Members>
     <AliasProperty>
       <Name>Modified</Name>
       <ReferencedMemberName>LastWriteTime</ReferencedMemberName>
     </AliasProperty>
     <ScriptProperty>
       <Name>Age</Name>
       <GetScriptBlock>[math]::Round(((Get-Date) - $this.LastWriteTime).TotalDays)</GetScriptBlock>
     </ScriptProperty>
     <NoteProperty>
       <Name>ItemType</Name>
       <Value>File</Value>
     </NoteProperty>
   </Members>
 </Type>
</Types>

3. Save the file at a convenient location.
4. Back at the Terminal (or the PowerShell Integrated Console), enter the following command to update the type data using the XML. Notice the PrependPath parameter.

PS> Update-TypeData -PrependPath ~/Documents/code/github/powershell/chapter-06/CustomTypes.ps1xml

5. Now, list out all the files within any directory of your choice.

Get-ChildItem . -File | Select-Object Name, Length, Age, Modified

You should see the new properties that you created.