It is fairly simple.
1. I prefer to sew on loops of 18 mm webbing to gromets. Be somewhat generous in size and you can put a tent-peg inside the loop with no problem.
2. Use reinforcements with all attachement points, I use a piece of the fabric placed at
45 deg to the main fabric. Something like 10-15 cm squares for center points, and
triangles of the same size along the edges, usually works for me.
3. For center points I put one patch on each side, and a piece of webbing sewn to that.
4. Roll the edges (so that you have three layers), pin, place the attachment point patches in place, and then sew it all down. Attach the points last.
5. Seal all holes with seam sealer or possibly Liquisole (be generous).
6. As to where you want your points... I find that about every 50 cm or so along the sides is good and flexible, and honestly only one center point for my style. I suspect that if you have a large tarp and want to have it over a hammock you may want to add one more along each diagonal (but the hammocky people can advice you better there).
7. Size? Not less than 1.5 x 2.5 m, I like a 3x3 m, but is thinking about making a 1.75 x 3 m; less versatile and less space, but lighter.
8. Buy a piece of bungy cord (say 50 cm or so) and tie in a loop. Attach to the center point with a larks head when in use and tie lines to this. This saves the fabric from the worst of sudden jerks if the wind is gusty.
