firstname.lastname@example.org) if you have any specific questions on that design, or building tarps in general. I'd be glad to help with any detail questions.
As for webbing loops - Grossgrain does work, it's just not as strong as actual webbing. If you do go with gg, I recommend you double it up for the ridgeline tie-out as that sees a lot of stress compared to the other tie-outs.
The most difficult part of building a tarp is simply the sheer volume of material you have to work with. And I suppose for some the slipperiness of Silnylon can be difficult at first - but it becomes second nature after a while.