Simple Quests is a minecraft mod that provides the ability to create server side quests. Thus this mod is not needed on the client.

The quests are defined via datapacks.

You can see an example of a quest datapack under https://github.com/Flemmli97/SimpleQuests/tree/HEAD/Example-Datapack Or in any other version branch under ./Example-Datapack

It is recommended to use the datapack generator under https://flemmli97.github.io/misode.github.io/ (based on this) which makes the whole process trivial.
Then go to Modded Generators -> Quests or Quest Category

Report bugs with custom generators under https://github.com/Flemmli97/misode.github.io!

Updating to 2.0.0

2.0.0 introduced various breaking changes. You can use https://flemmli97.github.io/misode.github.io/simplequests_convert/ to convert your old datapacks to the new formats