You can start the appraisals in Jan . Its entirely upto you. However, the hikes are generally offered with the financial year. Hence you might need to automate it from April and pay for the last three months with the new salary package.
There are two different forms which the employees need to fill. First one about the self-appraisal and the second one is about the reporting manager's feedback.
You can allocate marks to each question and then the sum-total to find an average.
You have to allocate a range of marks, such as 0-20, 20-40, 40-60, 60-80 and 80-100, for each rating. There are generally 5 ratings, 1 being the lowest.
Since this is the first time, please avoid using the bell curve. You can include only when your budget needs to be validated on the hikes offered.
There are too many forms uploaded in this community. They can be your frame of reference when you design one. Please share with us, more about your form, so that we can suggest and help you make it better.