What do i need to dye my hair silver/gray

I already tried to dye my locs that color before but it didn’t turn the slightest bit of the color i want and it went back to the blondish brown i had in before that because the person doing it only had dye and the bleach for it(after i told them what to get btw), what do i need/ what can i get that’s gonna 100% get my hair to the color i wanted