Here's a good tutorial: https://www.youtube.com/watch?v=TUuyUBdFqfg
Your best bet is to make or modify slip covers.
Proper re-upholstery is complicated and costs hundreds of dollars for a reason.
Cheryl is right. Re-upholstering involves removing the old coverings and inside stuffing, cutting new pattern pieces, then sewing and installing on to the couch frame. An industrial sewing machine, specific upholstery tools, and knowledge on how to use them correctly is required. There is a valid reason why upholstery services charge what they do for their work.