Pick baby up and hold her with one hand on your front with her legs around your waist.
At this point you have baby’s weight secure with the shoulder straps you are holding so you can leg go of baby and reach around with your other hand to grab a strap.
Cross the straps, trying not to let them twist for your comfort.
Once you have crossed the straps bring them back around to tie either under baby’s butt (my preference as I think it provides more weight support) or around their back (might be better for a smaller baby).
TIP: Baby should not be hanging low but sitting up higher on your chest. This may require you to pull on the straps a bit and give a little wiggle to get the carrier up higher. Or push baby and carrier up a bit with the one hand before reaching around to the back to cross the straps.
TIP: To ensure that baby is up high enough and that the carrier is tight enough to be extra comfortable, I have found it helpful to give a little bounce as I pull the straps together and the knot tight.
BACK CARRY
You might feel more comfortable doing this while standing up against a table or a sofa. Tie the carrier around your waist with the body hanging down behind you.
Pick baby up and hold her on your hip.
Lean over as you slide her around to your back and get her positioned. You can also have an older child stand on a sofa or table etc. while you back up to them hoisting them on your back.
Still bending over keep one hand on your child while grabbing the carrier body with the other hand.
Pull the carrier up through baby’s legs and over her back placing the 2 straps over your shoulders. Still bending over grab the straps, one in each hand and pull them to take up any slack in the carrier (you might also feel to ensure the carrier is not folded under baby’s legs and that her legs are not stuck in the fabric).
Holding the straps firmly stand up. At this point I like to pull the straps up and give a little wiggle to take up any extra slack in the fabric and ensure that baby is snugly down in the carrier. 
Bring the straps over your shoulders keeping them snug and either cross them on your chest
or bring them under your arms and around the front.
You can either tie the straps over the bum and legs, under the bum and legs, or behind the back. Each distributes the weight differently and some prefer one way over the other.
