Doing it at home is risky and it is difficult to predict the results, especially if you have coloured hair previously. Natural hair bleaches easier, while the part that was coloured before, even if it was a long time ago, will most certainly be different colour.
I went light ash blonde from dark brown hair that was coloured red before, but it is risky. My best advise is to visit a salon, unless you are ready to experiment to find a way to do it perfectly with your hair, but be prepared it might not be a fairy tale ending the first time, so don't try it before a special occasion.
First of all there is no bleach that doesn't damage hair. If you want to go light blonde from dark brown, it will do damage, be prepared to do a lot of conditioning. You will need to bleach it twice with a strong peroxide as hair goes orange or dark yellow the first time you bleach it and no amount of blue toner can neutralize that. If you are brassy orange applying ash toner will most likely turn you greenish-grey - not the result you want.
platinum blond after 2 bleaches (20vol or 12%) and silver toner in a daylight
What you need to bleach hair
Peroxide. I used Wella Cream Peroxide 30vol (9%) to pre-bleach my hair the first two times. This strong solution is strictly not recommended to be used directly on scalp. From what information was available on the web, 20vol is the strongest you are supposed to use directly on skin. My scalp was a bit sensitive for around a week after that, but other than that and unpleasant stinging sensation while bleaching I haven't had any problems with it.
Bleach powder. I used L'oreal Platifiz Bleach (blue powder). Be very careful not to inhale it while mixing - apparently bad things happen to those that do 🙂
Toner. I used Wella Color Touch Sunlights /18 (Color Touch was mixed 1 to 2 with 4% Emulsion) followed up by Wella Color Fresh Silver - Violet 0.6 and. Toner will help you to neutralize yellow/brassy tones after bleaching and deposit a light colour. It does wash out in a while as it doesn't penetrate hair, just leaves slight highlight on the surface.
Wella Color Touch Sunlights can then be used to deal with re-growth: applying it to the roots first, then to the whole head after that. It lightens hair by 2 - 3 tones and deposits a bit of ash colour on the hair, so it is nice to knock the depth out of dark roots and to refresh the ash tone while not bleaching the hell out of hair each time with strong peroxides. I usually keep it on the roots for 20 minutes, then apply to the whole head and let it develop for further 20 - 30 minutes. To neutralise brassy tones that might be leftover I follow up with Wella Color Fresh Silver - Violet 0.6. This works almost like silver shampoo, which you would use on blonde hair to keep it crisp between colourings, but it leaves a lot stronger blue-violet tone and doesn't wash away as quickly.
How to bleach hair
Bleach until you get to very light yellow colour - approximately as light as the inside of a banana skin. First bleach once and if you haven't got light enough colour, bleach again if you think your hair is in condition to take it. If hair feels a bit like chewing gum, slightly rubbery when wet, it's too damaged to take another bleaching and will simply break if you do it again. If after the first time hair is still too yellow and you need to repeat the process, first start with the most yellow parts, then apply to the rest of the head. If hair was coloured before, most likely it won't come out even no matter what the first times you bleach it.
How to apply colour/toner to bleached hair
Wash out the bleach thoroughly, towel-dry hair and apply violet toner to eliminate any yellowness. If yellow is darker than inside of a banana skin, it will probably not turn out completely platinum after toning it either. If you leave violet toner for too long, you will end up with blue-violet highlight in your hair, development time is different depending on how damaged and porous your hair is - it is best to check at regular intervals by wiping one hair strand dry to see what colour it is. I found 5 minutes was enough, althogh some people might need to keep it a lot longer thant this. If your hair was coloured before, you will most likely end up with brilliant platinum blond roots where hair was natural and a different highlight where it was coloured. You can apply slightly darker toner to even out the difference between the roots and the rest and keep conditioning your hair until you are able to bleach it again. In the meantime ash or violet ash blond toner or a semi-permanent hair dye is a good option. However, if your hair is very yellow (darker than a banana), you might end up with green-ish highlights when applying ash colour.