First you need to make sure you spend enough time - in advance - giving people information about appraisal, what it's for, how it's going to work, and most importantly, how it's going to affect them personally (i.e. how ratings affect promotion, reward, development etc).
There also needs to be a clear follow-up to the appraisal - what happens afterwards, any further meetings or actions, setting objectives etc.
You will also need to brief line managers in depth so that they can answer any questions and understand their role in the appraisal process.
Once everyone has been briefed, you will need to give people enough time to plan, schedule and attend their appraisal meetings, and then complete any follow up notes and actions.
On this basis, we would recommend that you allocate at least 2 months to the appraisal process.
If you'd like our check list for a successful appraisal, you can download this by going to
and clicking on 'Great tips for successful appraisal'.
However this doesn't include the design and validation of the appraisal: if you are starting this from scratch you will need, we would suggest, at least 10 working days to develop a pilot version and a couple of weeks to test it and validate it with key stakeholders in the organisation.