I have to use the couch or the bed. Bed's the best for me.
I lay the carrier out...waist straps out to each side and the shoulder straps hooked up and ready. I usually wear baby facing away from me when he's on my back, so I lay him on his tummy, sit and wrap the straps around my waist and then pull him on like a backpack. If it's not comfortable, I lean forward and wriggle till he's in the right spot. Sometimes we have to wriggle more than others.
Try rolling the waist straps part up once or twice before putting it on. (makes a seat for baby so he doesn't hang from the shoulder straps)
Do you have the chest strap for your trekker? That might be helpful, too. It pulls the shoulder straps together so they don't slide off the shoulders.