Turn ON power and increase the current until there is a free flow of gas bubbles from the parts. A minute or two of 'reverse plating' is usually adequate. Longer re-activation will begin to dull the finish to some degree. The plating bath temperature is not critical. It is only important to make sure that you are above the recommended operating temperature.
Low temperatures promote blistering and dull plating.
Make sure that you have reversed the connections to the power supply so that the part to be plated is now connected to the negative side of the supply and the positive side is now connected to your pure nickel anode. Allow a few seconds for the part to heat up to the temperature of the solution before turning on power. The plating solution must be under constant agitation while plating in progress. The easiest way to do this is to stir the solution with the part being plated. Also rotate the part for more even plating.