Willow is very fast growing tree. That makes the wood weak, but the strong bark keeps an integrity to it. Harvest the red shoots, they are usually the best ones. The shoots that are still red has a large pith, but the bark is strong, so that they can be folded without breaking.
Harvest in winter, when the sap is down, the shoots seems to be less brittle then and their water content is lower. But still, dry them before you use them, to avoid the baskets becoming loose. It is better to reconstitute them afterwards, before using them. That wasn't neccesary with these shoots though, as they were so strong. When harvesting, there is no need for tools, the easiest method is just to rip them off the trunk with your hands.
To start the weaving, overlap four willows like on the photo. But four will not do it. It has to be an uneven number, otherwise the weave will end on the same place and that will not make a basket.
When you feel the gaps between the spaces become unmanagable, add in an even number of new strands. If you add in an uneven number, the result will be even and will not work (Said differently: Not 5+5=10, but 5+4=9.). Cut off all of the ends on the inside, as that will be hard to do on the finished basket.
When you have the size of the bottom you want, bend up the strands and tie them together in the top to keep them like that. Start weawing upwards. It is messy in the start, but if you are careful to keep the weave tight it will become progressively easier as you move upwards.
Weave it as high as you want it. When the top has become stable on it's own, you can release it in the top, that will make further weaving easier. Splice on the outside, as trimming later on the inside will difficult.
Cut off the tops at a good lenght. Make them pointy, that will make tucking them back into the weave easier. Tuck them back over into the neighbouring strand.
Finally, trim off all the ends and the basket is finished.