1st step: What format is your original footage?
2nd step: Create a new sequence. Go to sequence > settings and in the bottom left corner click Load Sequence Preset. Choose the preset that matches your source footage. For example if it’s DVCPROHD 1080i60, choose that.
3rd step: Select all your footage in your original timeline and paste it into the new timeline you just created. It will look distorted.
4th step: Double click on one of your clips and in the Viewer click on the Motion tab. Adjust the scale to 100 and in the Distort drop down adjust the Aspect Ratio Slider until your footage looks appropriate.
5th step: Once that clip looks good, right click on it and choose Copy. Select the rest of your clips and choose Paste Attributes. Check the boxes Basic Motion and Distort. The rest of the clips should look good.
Obviously if you have different types of footage in your timeline this process will take more time.