get a 5 string leave the B, replace the E,A,G,D with heavier gauge then tune it thickest to lowest the same as guitars, tune up for the B to the C, then down for the rest that will sort out the slackness and the tuning our bassist uses his 5 or 6 string like this for when we play in C#/F#/B/E/G#/C# and B/F#/B/E/G#/C# also i you tune up its gonna be in the same octave as the guitars