The cheapest way is by using castings cut from 'trees' of castings such as those from PM Research and others. When you cut them off you get casting chunks like this:
Once cut off I use a little fixture like this in a collet to hold the fitting:
While chucked you can face, drill and tap. Pop it out, flip it around and do the other side and you can be sure that it's pretty square.
You generally have to make the fixture for each 'batch' of castings, I've had the supplier change their patterns and the fixture be just a bit loose. Once you've made a couple it goes pretty fast.
If you get the trees from a supplier who also furnishes finished fittings you can avoid making a fixture for a single casting, or trying to get a decent 45 degree angle when holding something in a 4-jaw.
There are better looking castings available out there (SuperScale, RR Warehouse, etc.) but while they look great they're too rich for my blood. The ones above are older ones from PM Research.
You'll also want a bunch-o-unions. I buy them, and like the solderable ones. The threaded ones are fairly hard to get sealed and it always feels like I'm going to break one when tightening them on the steam line.
On edit: The top picture is showing a couple of casting styles, I believe they're old and new from PM Research. The fixture idea came from an article in Live Steam some years ago, I can't recall the issues but it's simple after seeing it.
