There are two ways to go into Safe mode: you can Ctrl + click on your Outlook shortcut or type outlook.exe /safe into the Start search field or Run command. (Use Windows key + R to open the Run command).
When you hold Ctrl key and click the Outlook shortcut to open Outlook, you'll be asked if you want to start Outlook in Safe mode:
Outlook has detected that you are holding down the CTRL key. Do you want to start Outlook in safe mode?
Additional Safe switches you can use with Outlook 2003 and up are:
/safe:1 Starts Outlook with the Reading Pane off. Outlook 2003 and up.
/safe:2 Starts Outlook without checking mail at startup. Outlook 2003 only.
/safe:3 Starts Outlook with extensions turned off, but listed in the Add-In Manager. Outlook 2003 and up.
/safe:4 Starts Outlook without loading Outcmd.dat (customized toolbars) and *.fav file. Outlook 2003 only