I'm putting down a deck of 1x8 rough sawn douglas fir boards that I milled last year. They're 16 feet long, so each board runs the whole length of the shelter. Some galvanized ring-shank nails and it's a solid platform for the next step, the verticals.
I first notch the skid log so that there's a flat surface I'm basically cutting the curve out of the log. Each of these cuts is the width of vertical member that fits into it. It's pretty quick to do. You place your vertical there, scribe a line, and then make the cut with the chainsaw.
Then you do the same sort of cut to the vertical piece. Here I've cut a flat spot on this post.
If I did it right the post fits snug in its new notch. 6 spikes later and it's secure. I've been thinking about putting in a lag bolt here, but I think I'll defer that until near the end of the project. I think it'll be sturdy enough when I get the diagonal bracing in; the wood notch helps keep it in place, too.