In order to create a realistic and achievable project timeline there are two tools that can assist you:

A Critical Path Network Diagram
An Agile Gantt Chart

Critical Path Network Diagram

There are two forms of Critical Path Network Diagrams: The Activity on Arrow (A-on-A) and the Activity on Node (A-on-N). I prefer the Activity on Node diagram as is illustrates the activity dependencies and allows you to work out the critical path (the path that needs to happen on time in order to deliver your project on the due date) and the activities which have more time based flexibility. This process helps you calculate the earliest start and end date for each activity and the latest start and end date for each activity.

This video (7:25) shows you how to create and A-on-N diagram for your project.

Once you have your Critical Path Network Diagram it will make setting up your Agile Gantt Chart a lot easier!

Agile Gantt Chart

This video walks you through how to create your Agile Gantt Chart in your Project Planning Handbook. You may need to watch this a few times to get the hang of it (I know I had to record it a few times to get it right!). Having the drawing of your Critical Path Network Diagram for all your work packages will be helpful in setting the start and end dates for each task on your Gantt Chart.