And what exactly are you having problems with?
You could poll the undocumented chatters endpoint tmi.twitch.tv/group/user/<ChannelName>/chatters to look for new users, when there are new users in the channel check if they are subscribed using the https://api.twitch.tv/kraken/channels/<channel ID>/subscriptions/<user ID> endpoint (https://dev.twitch.tv/docs/v5/reference/channels#check-channel-subscription-by-user) and then if they are send the greeting.
Personally though I wouldn’t recommend using a feature like this for many reasons, such as some users just like to lurk in a channel and not be announced when they join, the bot might spam a lot if a lot of subs join at once so you would need to limit/queue the greetings so your bot doesn’t exceed the limit and get a temp ban, and if you have subs rejoining frequently that will potentially be a lot of API requests unless you also implement caching on your end.