Hello Harry,
The following is the brief explanation of the different options.
Should I check Only restrict to posts?
Use this option if you want to delete posts based on published date. If you don’t care about when the post was published then you don’t have to choose this option.
or should I check Only delete first?
Check this option if you want to delete only the first x number of posts. This will be useful if you have huge number of posts to delete and your server is running out of memory. If the number of posts that you want to delete is less or if you are not facing any issues with your server then you don’t have to choose this option.
should I put NOW on Schedule box?
The scheduler will run based on the date and time that you specify in this field. If you specify “NOW” then the scheduler will run immediately. If you choose some other date and time then the scheduler will run at that time.
Hope that explains most of the functionality. Let me know on what condition you want to delete posts and I will let you know how you can setup the different options.