A sofa slipcover seems like a daunting task, but if you think of it in small steps you can definitely tackle it!
If you’re a beginner I would suggest using an inexpensive fabric or even a drop cloth for your first attempt at slipcover making.
Drape your fabric over the inside back of the sofa and allow the fabric to drape down over the deck(seat of the sofa, under the cushion), and down the front. You may need to seam together two or more widths of fabric to cover the whole inside back and the deck….