How can you define the KPI's?

I will consider that we are speaking about business KPIs.

KPIs are a means for monitoring and evaluating certain metrics which support the success of your objectives which eventually coming out from your organizational vision/mission and Goal.. 

so to clarify more, i will put them in some logical order:

1- you should first have clear and S.M.A.R.T objectives in place , driven from your organizational Goal/mission.

2- then you need to decide what CSFs (Critical success factors) should be happen to be sure that your objectives will be realized. ask yourself what factors are critical (should Happen) to make my objective succeed.

3- after you state every CSF for you for every objective , choose the metrics which can be used to be measured, monitored and evaluated , and decide for each metrics, how you will measure. if there is some metrics can not be measured, then it's not a correct metric at all.

4- out of those metrics , choose from 2 to 5 key metrics (maximum) which are most critical and impossible for your objective to be reached if not happen , those 2 to 5 metrics are your KPIs which need your intention and close management.

I hope this can answer your question.

After declaring the strategy plan phases , you can determined the necessary project to complete every phase according to planned time schedule, here only you can define the planned KPI's to be completed through the execution plan, so you can compare the achieved with the planned to know where are you from the time schedule and to measure your executive activities.