Hair history:
My hair is naturally wavy
Dark brown
I straighten my hair roughly every 2 months with hot iron
I dye my roots only with natural instincts boxed dye to cover greys every 3 weeks and leave on for 45min- 1hr (any shorter and the greys don't dye)
I want:
To dye my hair sombre or something like pictured.
Questions:
Do I have to stop dyeing my roots before lightening? How long?
How many processes of treatment until desired look?
How long to wait in between lightening sessions ?
How often should I touch up to maintain?
Attached is what I'm at currently (dark) and where I want to be
Questions:
Do I have to stop dyeing my roots before lightening? How long?
No, because you don't appear to need to lighten the roots. But you should never bleach hair that is freshly washed, so make sure you have dirty hair, at least a couple of days, before bleaching.
How many processes of treatment until desired look?
No idea. It's different for everyone and you don't actually say how you're planning on doing it.
How long to wait in between lightening sessions ?
If it's full bleach, a month. If it's a bleach bath a week.
How often should I touch up to maintain?
It depends if it needs it. You shouldn't need to "touch up" lightened hair except perhaps toning. The it depends how often it fades.
Really though, you're not asking the right questions. What you really need to do is use a colour remover on the parts you want lightened first. Never go straight in with the bleach or a blonde hair dye. They aren't made to remove hair dye. This is the first step. Once you've done that we can help with the rest. I'm sure you've got questions about that, so feel free to ask.