Thank you for your question, Karen.  The answer is a bit more complicated than a one-sentence one.  I suggest that you read the following comprehensive article about blackberries, especially the parts devoted to "trailing blackberries", of which the Marion is one variety: https://extension.oregonstate.edu/catalog/pub/ec-1303-growing-blackberries-your-home-garden

You'll find the section on pruning (and training) this variety about 2/3 of the way through.  I hope this helps.  Good luck!