A power supply maintainer is a battery charger, but more. It holds steady the voltage at 13.8 which is needed for programming later vehicles. A drop in voltage can either cause the programming to fail and potentially brick your module. The current BMW recommended unit is the Deutronic DBL-800; it's in the $1200 price range (available via API-NET or Baumtools in the US). There are other options, but I decided upon a Midtronics PSC-550 (readily available, cheaper, and has similar specifications), and am waiting for it to arrive.

Forgive me, but I'm going through this out of memory, so I apologize if am not recalling screen for screen.

In SSS you go choose new session, choose the appropriate series. After the ZCS (in your case) is read, choose Load SW. SSS will ask if any modules have been replaced. Choose no, then you will have a list of modules. Select EGS, then choose Programming. SSS will query your EGS, then come up with a measures plan. There will be an old software version and a new software version to be installed. In my case the old is 7549958 and the new is 7564921.

The length of the measures plan will be displayed and then you can start on programming.

For an example, here's the list of times that SSS displayed for my updates:

DME - 60 minutes
DSC - 20 minutes
EGS - 20 minutes
VTG - 4 minutes