Here are some great videos to demonstrate how you get a baby on your back depending on your carrier type.
Soft Structured Carriers are great options for back carrying. The waist belt can offer a lot of back support, and padded straps can make for a comfy wearing experience. Once you have the waist strap secured around your waist, you can use a "hip scoot" or a "superman toss" to get baby into the carrier on your back. Watch this video to see how it's done.
Woven Wraps are incredibly useful and versatile when back carrying. With some practice, you can get baby high and snug on your back. This can alleviate a lot of the discomfort that comes from having them too low or far from your body in other carriers. There are numerous back carries you can use with a woven wrap. A Mei Tai is a lot like a SSC (soft structured carrier), but has the great comfort qualities of a woven wrap. For both a woven wrap and a mei tai, you can use a "hip scoot," "santa toss," or a "superman toss." The video below gives a great rundown of these methods.