The best way is to go gradually.
Stripping the colour with a colour remover like colour before is a good ideal first step to see how light you can get, with minimal damage. Depending on how much you have dyed your hair black before will be the factor in how light it goes. I know with experience i only get to a mid brown max, at a push.
After this use a NON-permanent 6-8 wash Nice and Easy and leave for minimum 72 hours. Any dying after a colour remover could result in it going straight back to black.
It may be that you need
a)another colour remover
b) a full head bleach
If its the latter this should be done with weak bleach as its a full head application and could burn your scalp.
There are also professional products that lighten the hair and deposit the red in one go, but these can be quite damaging.
The trick to maintaining red is making sure your hair stays in good condition, or its pointless. Applying red to damaged hair will just not last, and the red will come out quick.
Its a big change, and if you dont know what you are doing, speak to a hairdresser, who can see if your hair will sustain. Hairdressers also have access to colour strippers (not to be confused with colour removers) and they give good results, but this can take a few hours in the salon and lots of money!