1) You need Garmin Mobile XT for Windows Mobile, the newest version. Its discontinued now, but its still available from the Garmin Website. Download the setup application, and get the Basemap and Support files.
2) Once you have the three setup files on your desktop PC, connect your mobile phone via Activesync or device manager, and get them synced.
3) Install the Mobile XT application. You can install the support files (which has voices, delete the voices you do not need as this will save space on your memory card). Install the basemap (without the basemap, the map image you intend to install will be much slower to use).
4) Run the Mobile XT application. You wil be given a number of options, like registering and such. Just choose the option at the bottom (connect to GPS), and just keep selecting things until you get to the application itself. Since its not registered, it will not allow you to connect to GPS, but go to Settings, and select About tab. This will give you the device code - note down this device code (nothing else) and then exit the programme.
5) You need Garmin Keygen. Install this on your desktop and use the device code you obtained from step 4) from the About settings on Mobile XT. You will get an access code - just copy and paste this onto a notepad file, and save this as SW.UNL (make sure you save as 'all other files' and not as a .txt file or it will not work).
6) Take this SW.UNL file and paste it in the Garmin folder on your mobile phone. I have found that some Windows Mobile devices will only work with the .UNL located in the Garmin folder on the phone, not on the memory card, hence for simplicity, just install the .UNL file on the phone. Run Garmin Mobile XT, and it will work unlocked.
7) Copy your map IMG file (GMAPSUPP) onto the Garmin folder in your memory card. Ensure your map is unlocked for use. Garmin Mobile XT cannot use 3d Buildings or Junction View, hence don't bother copying these to your memory card.
8) If you are using an older device with a slower processor, consider using an overclocking application for your device, as this helps speed up the application. Its dependent on what processor your telephone or device use, but always back up your device before routinely using an overclocking application. Garmin Mobile XT runs on most Windows Mobile devices, even the old Pocket PCs, but you may need to use the older version of Mobile XT for them (try the newest version first, if it does not work, then try the older version).