Reupholstering furniture involves replacing both the covering and the inside foam or stuffing. Recovering furniture is removing the outside fabric, and then attaching new fabric covering on to the chair frame.
To complete a reupholstering job correctly, many different specific tools, an industrial sewing machine, and knowledge on how to use these is required. There is a reason why upholstery shops charge what they do for such work.
As far a recovering a chair yourself, the difficulty of the work really depends upon the style of the chair, how much fabric there is, and how many pieces there are in the pattern.
Could you post a picture please of your chair so that we can see what you are starting with?
The best way to make a new pattern is to remove the old cover and gently take it to pieces, then use the old pieces to make a new pattern. Use a flat stapler (about $12) to put the pieces back together and attach the new cover to wood parts. Best wishes!
To get visual aid, look on you tube. Lots,of videos there on that subject.