Slice or cube the watermelon meat.
Add it seeds and all, to your blender and pulse until it is liquid.
Pour it all through a mesh strainer into a bowl or measuring cup.
Constantly move your spoon back and forth to disperse the pulp so that the juice can flow out through the bottom.
Discard the pulp and seeds that are left in the strainer and repeat, repeat, repeat until you have made all the juice you want.