the best way to re string a metal venetian blind is to select a replacement cord that maches the existing cord as closely as possible. never choose a thicker cord as this can stop the blind from working.
measure the blind to estimate how much cord you will need, for example if the blind if five ft drop and has two wholes for cord and is four ft wide you will need appx 25 ft of cord
if the blind is still corded you can attach the new cord to the old cord and pull it through the mechanism, simply attach the new cords to the old cords using sticky tape then carefully pull the cord through the bottom of the blind taking care to enshure that the cord lock is relesed.
to relese the cord lock on a venetian blind you simply pull the lift cords to the center of the blind.
if there are no lift cords in the blind at all i recomend looking at a blind that is still corded and useing that to refer to.
if you are recording a blind on ladder strings it is essential when feeding the cord down through the blind that you step the cord, alternate the cord down one side of the ladder then on the next rung on the other side in a left right pattern as you look at the blind, this will stop slatts slipping out of the ladders when the blind is raised.
to lubricate a blind never use oil as it attracts dust, try useing a silicone based furniture polish (spareingly) on any of the moving parts in the headrail at the top of the blind