I've recently posted some Learning Guides to practising Diminished and Dominant Sevenths for ABRSM Grade 8 piano. I felt that students needed a framework for practice: so that they could see more easily the common elements in some of these arpeggios, and the differences in others.
I find the main problem with playing arpeggios with a seventh in them - either diminished or dominant, is getting used to using the 4th finger. Every arpeggios learnt for Grade 5, (that is all of them!), only has three notes in each octave.
Start with this Hanon exercise, one hand at a time, initially: pdf logo (click through to Hanon diminished 7th exercise)
Guide to playing Diminished Sevenths on the piano
Then follow this Guide to Playing Diminished Sevenths:
Guide to Playing Dominant Sevenths on the piano
Finally, tackle the Dominant Sevenths here: