How would you do it? (Dye question)

I honestly wouldn't use the cosmic purple. It will fade in a couple of washes to a similar shade to the mahogany brown.
What colour is your hair now? you will get a longer lasting result by applying a direct dye over a lightened base. If you want a dark/muted colour (similar to the box purple) this is possible, you just don't lighten the base to pale blonde.

I've already bleached it to lighten, currently I'm letting it rest, but it is very very light compared to what it used to be :]

If you've bleached, then you shouldn't use the box dyes anyway. Especially if you bleached less than 4 weeks ago. I would look at the semi dyes on Beeunique instead.

Then, for applying, section off the area you want purple, and put the purple on it. Wrap it in cling film, and then put the brown on the rest.

I would look at the Adore browns, they have lots of choice, they are non damaging, long lasting and really cheap. Adore have some lovely purples as well. http://www.beeunique.co.uk/index.php?main_page=index&cPath=151
You can use these straight away, no need to let your hair rest.