I was told you could use seaweed as fertilizer in the veg garden, so I got myself a ton of it. Do I wash it first and then put it in the base of the drill before planting potatoes etc or how is it applied?
It is mostly water and rots away to very little, but it does provide important minor nutrients and trace elements. There is some salt in it, which favours some crops such as asparagus and beetroot, but not enough to do any harm to others. So no need to wash it.
In general, a good way to use it is to stack it in layers with general garden compost and mix it through as the heap is turned or dug out.
If you have a ton of it, spread some over the soil and dig it in, or place some under potatoes in drills with soil on top, and stack the rest for use later. It will breakdown to very little volume.