I've got a book from my university library - "Rudiments of the Flute, Recorder and Oboe" by Jacques-Martin Hotteterre.

It mentions (for tonguing) the syllables "tu" and "ru", but I'm not sure how to pronounce them.

Is it "tah" and "rah" or "too" and "roo"?

And I'm aware that the "ru" should sound something like a "du" but I'm not really sure.

I know things like pronounciation are difficult to explain through text but if anyone wants to have a go I would really appreciate it.

As far as I'm aware, it's the French 'tu' and 'ru'.

If you don't speak French, then it's ths sound you get by putting your lips into an 'too' shape but saying 'tee'. For the 'r' of 'ru' I think your tongue touches your palate further back than for the 't', with a tiny touch of rolling.

I go for a gentle action between tu and du, but you can go more towards one or the other if you want a certain effect - tu is brighter and bolder and du is more soft and gentle.