Single-V butt joint with root face

Set up the V-groove with a root face so that a small land remains at the root. The land prevents you from burning through on the first pass and lets you control the root penetration cleanly. If you leave the face too high, you won't get proper fusion; if it's too low, you'll burn through. When welding, make sure the root melts cleanly without the face completely melting away, otherwise you'll get undercut or cracks in the root area. So: prepare the groove cleanly, keep the face even, and place the first drop controlled, then it'll work.