A watch party is one of the lowest effort events you can host. There is a built in start time, a built in activity, and a built in reason for everyone to be there. That simplicity is exactly why the few things that go wrong tend to go wrong in the same predictable ways.
Be clear about the start time
The whole event is organised around a moment. The kick off, the first episode, the broadcast. The single most important thing in your invitation is when to actually arrive, which is not the same as when the thing starts. Tell people to come at least half an hour before, so nobody is arriving mid first half and stepping over everyone to find a seat. "Doors at 7, kick off at 7:45" removes all the ambiguity.
Sort the screen before anyone arrives
Nothing deflates a watch party like fifteen minutes of fiddling with inputs and logins while everyone waits. Test the stream, the channel, the cast to the TV, the sound, hours before. Know your backup if the stream drops. The host who has the screen ready and working at the door looks effortless. The host troubleshooting at kick off has lost the room before it started.
Match the food to the format
People watching a screen do not want a sit down meal. They want things they can eat without looking down: finger food, a big bowl of something, things to graze on through the breaks. Order or assemble food that survives being half ignored for two hours. The watching is the event, the food is fuel for it, so keep it simple and constant rather than a production that competes for attention.
Know your crowd's noise level
Watch parties split into two types and it is worth knowing which you are hosting. Some crowds want to shout at the screen and talk through everything. Some want to actually watch in relative quiet. Mixing the two badly ruins it for both. If you have a serious watcher and a chatty crowd, say a word about it, or seat them so the talkers are not behind the watchers. Setting the expectation avoids the tension where half the room keeps shushing the other half.
Keep the guest list to the space
Everyone needs to see the screen. That is a real constraint a normal party does not have. Twelve people around one television is fine, twenty five is a bad night for the people behind the sofa. Count the seats with a view and invite to that number. A watch party is one of the few events where capacity is set by sightlines, not by floor space, so plan to the screen, not the room.