Each photo must overlap a minimum of 15%. Use software like Adobe Photoshop or Adobe Lightroom. In Photoshop, open all the photos for the Pano. File > Photo Merge > Use all Open Photos.
The time to merge them, depends on your computer, number of photos and file size. After they are Merged, the new Pano will have an irregular shape. Crop it to you liking.
It's best to use Manual settings for Exposure. Auto Exposure can cause a difference in lighting from one photo to the next.
Shoot in RAW so that any change in White Balance can be corrected before the Merge.