Once you have a good idea where the file is, launch gmlw.exe, or if you're using the source version, type python gmlw.py.

The default SMTP server of gsmtp57.google.com works for most users.  If your ISP blocks outgoing mail to other servers, however, you will need to specify the settings for your own SMTP server.  Clicking the "Requires Authentication" option lets you provide servers with your username and password. Some people report that gsmtp57.google.com is not working for them. If so, try one of the servers from this list.

The second step is to configure your Email file.  Using the "Find" button, locate your mailbox file, or if you're using a MailDir, select any file inside your MailDir.  

Then, choose the type of file you just selected.  There are two versions of mBox, one which is more strict about the format of files and therefore more accurate, and one that is very lenient but necessary for some mBox files (To know why, read this note by Jamie Zawinski). If you don't know what format your mal application uses, try Googling for "mail format <your application name>".

You then need to choose the type of messages you're sending.  If you choose "Mail I Received" messages will come to your inbox, and appear to be from the original sender.  If you choose "Mail I Sent" the messages will be relabeled with your new GMail address, and will appear in your "Sent Mail" folder.  

GMail labeling automatically marks incoming messages as 'Inbox'. There's not a way for an external application to manage this (if Google would provide some X-Header control of this functionality and the time/date parsing it would be very nice) so messages imported into 'Sent Mail' Will get both labels, which means you'll see your sent messages in both places. There is only one copy kept, though, and the messages are relabeled so they appear to be from 'me' and not your old email address. If you 'Archive' the copy that appears in the inbox it will only appear in 'Sent Mail' (and 'Archive' of course). 

Finally, enter your entire GMail address.  

Once you click "Send to GMail", the application will start sending messages, one every two seconds.  The delay is necessary to prevent flooding of Google's servers.

Clicking "Save Log" will allow you to save the output window to a file for later review.

Clicking "About" will launch a new web browser window to the GML Homepage.

Some Issues:
The timestamp in the inbox will be the time the message was received by Google.  Inside the message the original date will be displayed.  You can search for parts of dates to retrieve matching messages.  "Aug 94", for instance, will show you all messages from August of 1994.

The count of messages in your inbox will not match the number GML reports as sent.  This is due to the fact that the number GML reports is the number of new threads, not individual messages.  Google automatically groups related messages as they arrive.

Some people, especially users of Mozilla or Firefox, report problems with their mBox files being corrupt.  I have tracked down a cleaning script that can fix most of these problems.  

Importing mail from Outlook is a bit spotty.  I recommend one of two things.  Either import your Outlook mail into Outlook Express and then into Thunderbird, or pay a little for a copy of PST Reader, which should save your Outlook mail as a mBox file.