Spellings are UK English.
When you've entered a word it will turn yellow if it's both in our lexicon and is contained in the word above it. That doesn't necessarily mean it's the right word - the wrong word early on will lead you to a dead end. Each puzzle has at least one solution using easy words (a typical 8 year old's vocabulary).
The format came to me in a dream; I literally woke up in the middle of dreaming about doing a puzzle in a newspaper.