basically, flashing a winmo phone is an easy process:
1. bootloader mode (power on with VOL- button pressed)
3. hard reset
however, there are a few preparations if your device was never flashed before, you need to install hspl on your phone and make sure you have the correct driver installed on your pc. the driver for the device in bootloader mode might vary from the driver used to sync the device, when switched on.
1. the driver:
if you have windows xp, connect the phone in bootloader mode and look, whether it changes from serial to usb on the bootloader screen, you should be fine by then.
on windows vista/7 you connect the device in bootloader mode, and might actually take some time for it to download another driver from windows update. if it changes from serial to usb, first try to connect using mtty, before attempting to flash, because the connection might not yet be stable and you might want to have a look at the thread (ref updating the usb drivers).
to quote the dummies' guide thread:
(HSPL2a) Place your phone into its bootloader.
(HSPL2b) Run HSPLWrapper_Blackstone_156OliNex.exe from the download in step (HSPL1a) and follow it as normal.
(HSPL2c) If successful, go to step HSPL3. If the flashing process sits at 0% and you get Error  connection after some time, go to step (Troubleshooting).
pretty easy, in other words, just connect the device in bootloader mode and run the package on the pc, it will unlock the spl and flash hspl.
if all went well, you are ready to flash, download any rom you like from the rom development section, and flash it using the .exe file that comes with them, might be updater.exe, blackstone RUU.exe or something else. if your rom download only consists of an .nbh file, like ruu_signed.nbh or BLACIMG.nbh, download advanced RUU and use it to flash.
alternatively, if this is to complicated for you as a beginner, you can rename the rom file to BLACIMG.nbh (if it is not already named like that) and put it in the root of the sd card, by then entering bootloader mode, it should find the rom and give you detailed instructions on how to install it.
last step, hard reset, as described in the original manual, switch on, while pressing VOL- and VOL+ and follow the instructions onscreen.