How to prepare for multiplayer integration?

I plan to add multiplayer to my game, but I have never done so before and do not really know anything about it. I have been putting it off until the game has more substance, but I worry that the more I add to the game, the more work I may have to do later to adjust everything for online multiplayer. I wonder, would there be a lot of adjustments to be made if I add multiplayer last? How much more difficult would that be than starting off focusing on online-friendly code?

Thanks in advance.

The general rule I always hear is that adding multiplayer to a finished game is nearly impossible, and you should absolutely start implementing multiplayer as early in the development as you possibly can.

Of course this depends on how complex your game is.

As above, unless it’s a very simple project, definitely design the game from the ground up as multiplayer rather than converting later

Secondly, if you have never made a multiplayer game before, start by making some simple test projects to get some experience with it before starting work on something more substantial.