Basic rules of thumb...
Have slow PC
Have DOS
Have RIB
Have software
Have brain
Have patience
Know SysID
Know TGID's
If UHF/VHF Know Base, Offsets, Start/Stops...
If your doing this for an ASTRO-25 system you MUST be a athorized user of the system as your radios will automatically affiliate to the system.
If your doing this for say just an Analog Smartzone/Smartnet you can program the unit to NOT affiliate, and disable the PTT features.
For an older "mixed mode" ASTRO system this can be done also.
And again, on an ASTRO-25 system you can NOT monitor without affiliating.
But basically you plug the radio into your PC vias a RIB/Cable and using the proper software for your radio/system type, you change the settings as you wish, and once the proper CC's, and other info are correctly loaded.. Your good to go.
Best of luck, and have fun!