This is a simple way I've found on youtube (see video at the end of the solution).
It worked for me, and I've done it using only one bucket to carry lava and water.
EDIT: After the Title Update 12 in the tutorial level you can find a bucket in the castle, and a lava pool near the sea after the castle.
In the Brewing Room there is a chest with a bucket:
On the other side of the castle you can see the sea, a lake, and a lava pool:
The video guide is made by gamersgonnaplay, not mine.