Do you plan on doing it out of the salon? If so, it's quite tricky to do yourself. If so it's probably best to get someone to help you. It's possible, but if you make a mistake you will probably have to either dye it all over, or get a professional to fix it. Not trying to discourage you! Just as a backup plan.
I suppose you would purchase a hair dye, and section off peices of your hair. Darken up that colour, and leave the parts you want untouched. You could use a hair the or a protective wrapping (cling film, tin foil etc.) To protect the bleached parts from the dye.
Condition and shampoo your hair with what it needs. This will depend on the condition if your hair. If it's breaking off, it will likely need protein. If it's dry and brittle, it likely needs moisture. This will be easier to determine on what it says on the label. If it says it's 'strengthens', or 'repairs', it's likely a protein product. If it's mentions 'nourishing', or 'moisturizing' or hydrating, it's probably a moisture based product.
Sorry if this is all obvious to you. I wish I could be of more help but I am not a professional. To deal with a situation like this is fairly straightforward, so you don't necessarily need to go to an expensive place to get it done. However if you really wish to do it yourself I would get someone to help you, and properly research how to section off the hair and apply the dye to the roots. All the best if luck and sorry for rambling!