That first square nail is a rite of passage, and rebar is honestly a decent choice for it since you've got nothing precious to ruin. One thing I wish I'd known starting out: rebar is mystery steel, so it can crack or get crumbly if you push it too hot. Work it at a nice bright orange, not sparking yellow, and you'll be fine.
For the taper, don't try to do it all in one heat. Pull it out in short bites, rotating a quarter turn between hits so it stays square instead of going diamond on you. Keep your hammer face flat and let the anvil do the work. The head is the fiddly part. Punch or drift a small hole in a block of mild steel, or just use the pritchel hole if your anvil has one sized close. Stick the shank through, upset the stub end into the countersink, and don't hammer too long or the head welds itself to the block. Pull it out quick and square it up with one light tap on the face. Trim the point last, when the whole thing's cooled enough that it won't wander. They never come out perfect the first dozen times, but they hold a board just fine.