The easiest way is to use a gas shielded mig and clamp a thick copper plate to the back side then weld the other side shut, remove the copper, weld a bit on the back then grind.
It is also easy and nice looking to tig weld them.
You can also stick weld with the copper piece behind, that works good to, but better when it is vertical so the slag runs off instead of puddling up in the hole. You can weld some, stop and remove slag and weld some more though.
Hope this helps,
Grigg
