As there are a lot of changes needed to send messages through satellites it is best to have a dedicated instance. To do this, simply make a new empty directory and copy APRSIS32.EXE into that directory. When you double-click the .EXE, it will prompt for new configuration as it originally did. If you're used to running the Development version, you'll have to set that again in the APRSIS32.XML file. It is perfectly fine to point multiple instances to a single copy of the OSM Map tiles.
You'll want to configure an APRS-IS filter to receive satellite-related traffic.
Set Configure / General / Add Filter to b/RS0ISS*/W3ADO-1/ARISS d/RS0ISS*/W3ADO-1/ARISS
You'll want to disable unnecessary things on the APRS-IS port.
In Configure / Ports / APRS-IS, uncheck Telemetry, Beacon, and Bulletin/Obj. You'll definitely want to uncheck IS to RF Igating since you don't want to transmit other people's -IS messages to the satellites. Leave that for them. Unchecking Beacon is a good idea so that your Beacons only go out RF and they'll only show up in APRS-IS if you were successfully copied by someone else.
In Configure / Ports / <YourRFPort>, uncheck Telemetry, Bulletin/Obj, IS to RF (redundant, but safer) and Me not 3rd (not sure why you'd uncheck this one, but it's a modifier on IS to RF anyway). Keeping RF to IS checked (both here and in the APRS-IS port) will send any RF-received packets to APRS-IS providing SGATE (Satellite Gateway) operation.
If you send APRS Messages via the satellites, make sure you uncheck Ack since you're not really likely t get any. This also eliminates the APRSIS32 automatic retries and leaves your transmitted text in the Chat box for manual retransmissions. You can monitor the APRS-IS stream on another instance to see if you've actually been digipeated by the satellite and gated by another SGATE (APRS-IS will never send you back your own packets).
You should also change the "Best" checkbox until it says "RF Only" and is checked. The "Send" button will also say "Send RF". In this configuration, any message you send will only go via RF and not via APRS-IS, but yet acks (if any) and incoming messages will still be accepted from both RF and APRS-IS.
it is Handy to have Enables / View Logs / <YourRFPort> window open and enabled see if you got through or not, and also your normal instance (with the same Add Filter shown above) to monitor the Packet Scroller (I often get through and am gated but don't see my own RF digi).
You will need to change your Configure / Beacon / Path depending on the Satellite you are attempting to use:
NO-44 works in sunlight only but is capable of transatlantic work.
You will read some recommendations to add SGATE to your path like ARISS,SGATE or ARISS,SGATE,WIDE2-2 but this usage is mostly unnecessary and only serves to make your packets longer which reduces the available bandwidth and decreases your changes of being copied. The SGATE was the trigger for dual-ported TNCs to gate packets received from the dedicated satellite frequencies to the local VHF APRS frequency with remaining hops in hopes of getting it subsequently gated to APRS-IS. Most satellite APRS monitors are direct IGates these days so the SGATE usage is no longer necessary.