Make sure you have v 1.20 of the Prog08sz software, newer versions have some configuration problem that no one has figured out yet and do not work. (Grab the good version of prog08sz.zip from yahoo or my site.)
For MegaSquirt code (not tomtek ignition), assemble both the boot_r12.asm and megasquirt.asm into their respective .s19 files. (Tom's ignition code and dual table MS code already contain the boot loader, so you only get one file.)
Start up prog08sz, you might have to retry several times to connect. If you can't get a connection, read the 908 archives for some hints on debugging.
Often the connect problem is low voltage due to a wimpy serial port, so Jim says eliminate this problem by connecting an external power source. (e.g., a 9 volt battery) to the circuit. Connect the + to the banded side of D2 (or D1) and the - to a ground point (e.g., the mounting lug on the 9 pin connector).
Select the 908_gp32.08p module when asked.
If this chip has been programmed before, you need to erase it (if in doubt, do this anyhow). Go to the menu on the right and select Erase Module. When this is done, it says "???", after which you should verify the chip with Blank Check Module.
Click on Specify S Record, and select "boot_r12.s19". Click on Program Module. After a few seconds, it should complete and you can then click on Verify Module.
Click on Specify S Record again, this time selecting "megasquirt.s19." Follow this with Program Module, Verify Module and you should be ready to insert the chip into a MegaSquirt controller and run.
Note: You can consolidated these last two steps by including the boot_r12 code when assembling megasquirt.asm (just uncomment the last line in megasquirt.asm before you assemble it). By default, the Dual Table code includes the bootloader, so you need only burn megasquirt_DT.s19 and no other files.
If you encounter "File access denied" errors while programming, make sure that the protection on Security.ini and 908_GP32.08P is NOT set to read-only, they should be writeable.