1) Clean the part. Do not use a grinder to removed spatter or slag. There must not be any coatings or things like grease on the area to be examined,
2) Apply the penetrant. The dwell time will depend upon a lot of things suck as temperature of the part, relative humidity and the brand and type of penetrant being used.
3) Remove the excess penetrant. Do not apply the cleaner directly to the part.
4) Apply the developer. Wait for it to draw the penetrant from any potential crack.
5) Interpret the results. Remember dye penetrant is just an aid for visual inspection, and as such the person doing the interpretation must be experienced and not be mislead by any non-revelant indication. Just because you have an indication does not mean you have a defect.