"Open Steam/config/loginusers.vdf in an editor such as notepad.

Find the account you want to force into offline mode (if there are multiple associated with this Steam installation). This should be easy because each entry has "AccountName" which is the name used to login and "PersonaName" which is the display name.

Set "WantsOfflineMode" to "1". 0 is false and 1 is true, so changing this to 1 will make it launch in offline mode.

4a. If you want to skip the "Go Online/Start In Offline Mode" dialog on startup add an entry below "WantsOfflineMode": "SkipOfflineModeWarning" "1" where the whitespace between the key ("SkipOfflineModeWarning") and value ("1") is 2x 'tab's. The tabs between key and value are required for the file to be parsed properly; without them it may fail, and Steam may ignore/invalidate the entire file causing the need to login again, fail to open, or etc. This entry can automatically be added by opening the Steam client and pressing "Go Offline".

4b. Change "SkipOfflineModeWarning" back to "1" each time you start Steam to continue to skip the warning."