My approach depends on just what I'm aiming for. Let me explain...

In the woods, maybe I will fell a tree and want to convert it before removing the log. When I do this, I will make makeshift stands out of branches or other large pieces of wood that are made on site. Usually, I will take some long branches and do something similar to what Richard describes above.

I will often cut notches into whatever support and roll the log into these. I have use log dogs, but prefer not to. I do not like to drive holes into my timbers.

Now I often also have logs delivered to a work site to hew and convert right there. Last time I did this, I selected 4 logs that were no good -bad knots or heart rot- and used them to make stands. I cut notches into 2 of the logs -2 notches in each log- then rolled other logs on top of them, perpendicular to the first. Then I cut notches into these top logs. This whole setup brings the logs I will be hewing up to about waste height. That's where I want them for my style of hewing, but if you use American style you maybe want them lower, so this setup won't work.

That setup makes a very sturdy assembly, where I can have 2 logs worked on at a time. Of all the things I have tried, this is my favorite.

When the logs are rolled into the notches and oriented how I want them with crown, etc, I use either a level or a plumb bob (whatevver I have on hand. I find a plumb bob to be more accurate, but takes a little longer to use) to mark reference lines on both ends of the logs, then I use a square to mark off of this reference line to establish timber profile. After that, it is a simple matter of connecting lines with a chalk line.