It depends on when you are travelling. After Oct.1 there is scheduled to be US customs pre-clearance. You will clear US customs in Halifax before boarding the plane. When you arrive in Newark you will stay in terminal C and do some shopping and have a little lunch, whatever you want as long as you don't go out through the security gates. Your baggage will be loaded without you doing anything. Fly to Orlando and either pick up your baggage or let Magical Express do it for you if you are staying at WDW onsite.
If you are travelling before Oct.1 (or if pre-clearance isn't operating yet) you need to fill out the customs form that the airline will give you on the plane and have it and your id ready when you land. Follow the crowd to immigration, you won't be able to go in the wrong direction, and wait to see the immigration agents in one of the booths. Then follow the crowd to baggage claim, retrieve your baggage and pass to the customs agent. There you will give the customs form to the agent and drop your checked baggage on to the belt for it to be loaded on to the plane. You should now follow the directions to security and be ready to show your id and boarding pass and have yourself and your stuff screened. Pass to the correct gate and resume thinking about your visit with the Mouse.
I flew through Newark last week and so this procedure is current, but it is much easier to hope for pre-clearance in Halifax, especially with short connection times.
To come home you just check in in Orlando and go through security once there, no need to deal with it in Newark as long as you don't go out the security gate, and clear Canadian customs and immigration in Halifax, which won't be difficult to deal with.
