Without the aid of a WFM tool, you will need to revert to an analyst’s best friend….Excel. It appears that you have quite a job on your hands but I will assume that you do have access to data from your ACD.
You will have to start in a reactive way. Build up a picture of historical data by interval that will show you certain metrics like service level, call volumes, ave work time, ave agents etc. This will allow you to identify areas of concern as well as provide you with the information needed to produce a forecast/manpower plan.
You should then be able to match the set shifts to the forecast in order to identify resource peaks & troughs and recommend schedule changes accordingly.
Obviously, this is a simplified answer to a complex task. Let your service level data be the driver to identifying resource gaps as a starter. Then you can worry about other factors like shrinkage, adherence etc.