essentially make it up as you go along
example:
45378.5
First digit was used to refer to the 24th century, but quickly forgotten. Basically when playing in the TNG era your stardates should start with a 4. Middle to Late DS9 and VOY should be 5.
Second digit refers to the TNG season. 5 would be season 5.
8 would be the first season after TNGs 7th, which would be DS9s 3rd season and so on.
Make the rest up as you like. Just keep raising the numbers continously with each ep. I usually use the last digit as a roughly 2.4 hour period.
I'm sure there are some (almost but not quite) sensible explanations in converting the stardates to regular earth dates. But I wouldn't really bother, it's not like the authors care about it. ;)