Whether you have a new arrival or a mature lotus that needs repotting, the procedure is the same. Get the largest round rubber or plastic pot you can find. Ideally, the pot will have at least an 18-inch diameter and be 6-inches deep. Square containers can cause the plant to die if a tuber gets crunched into a corner during high growth periods.
Lotus are strong growers, so make sure there are no drain holes in the container that would allow roots to get out.
Lotus can quickly rot in organically rich soil so it is best to use clay soil or potting mix made especially for pond plants.
Add soil until it is about 3" deep in the lotus container. GENTLY pick up the lotus tuber, as damaging the new shoots can quickly kill a lotus. The tuber should ideally be white to gray in color and be very firm to the touch. Hairy roots and new shoots will grow from either end. If leaves and/or pads are attached, trim back most of them. Gently place tuber on top of soil, taking care not to damage the new shoots. Slowly add more soil (again being careful of new shoots) until about 1" of soil covers the tuber.
At this point, if you have really sticky clay soil, you can place the lotus in the pond. If you don't have sticky clay, you'll want to add a layer of pea gravel otherwise the tuber will quickly float out of the soil. You may carefully put a rock on top of the tuber as long as it doesn't damage the new shoots.
If your pond is not yet warm enough (approximately 70°F), you can use dechlorinated water from your house to fill the pot to the rim. The lotus can remain in that container, in the sun, until your pond temperature becomes warm enough.