Assume the Position
“Place the soles of the feet completely under the thighs, keep the body straight, and sit at ease.” Siva Samhita, 3.4.95
Sukha means pleasant or easy. Sukhasana is often called easy pose or happy pose, and sometimes tailor’s pose. Learning to maintain this and similar meditation poses is, according to many yogis, what the physical practice of Hatha Yoga is all about.
To begin, sit on the floor with your legs extended. Fold one leg in, bringing the foot underneath the opposite knee or thigh. Then fold the other leg in the same way.
Feet

Another good reason not to sickle: if sukhasana becomes your meditation posture, it can do permanent damage. These are my feet (and sock-lines, thank you very much!). I spent roughly a decade sitting on my foot wrong in meditation before I got into Yoga and heard about sickling.
Seat
To do this pose with correct alignment, most of us need a meditation cushion or a folded blanket to sit just on the edge of. The purpose of this prop is twofold: 1. it allows the knees to be below the hips so the thighs release down, and 2. it preserves the natural curves of the spine. Both of these things lead to better posture and greater comfort.
With your legs folded, you want to feel that you’re sitting on your sitting bones, or ischial tuberosity, that protrude from the bottom of your pelvis. If you don’t feel them, rock forward and back until you find them. These and your tail bone should go down rather than be tilted under or out back.
Knees
Spine
Tuck your chin just slightly, feeling the back of your neck lengthen. You want to lengthen your spine while maintaining its natural curves.
Shoulders and arms
Second Side
So, after your designated hold time, extend your legs. Shake them out if that feels appropriate. And then fold the opposite one under first.
How Long Should I Hold the Pose?
Most of the time in classes, sukhasana is a base pose for other movements and not held too long as a pose in itself. When you do hold it, you may find that after a very few minutes your back starts to fatigue and you feel like slouching. That’s normal for people who grew up sitting in overstuffed couches and chairs with backs.
The good news is that nearly all the Yoga poses help make sukhasana easier. Practice sukhasana whenever you can and maintain a regular Yoga asana practice, and eventually you'll be able to stay in sukhasana for hours, if that's the sort of thing that's appealing to you.