Purging the data older than 1 year

I have a requirement to purge data older than 1 year.

Table has 5 billion rows and delete about 4 billion rows in batches.

there is no primary key or indexes on table.

There is column Datetime and i created nonclustered index on table and planning to purge data based on that column.

Any help in preparing the script to purge data in batches older than 1 year.

Thanks,

Ron.