You want it thin enough that it gets limp when soaked, deer leg sinew thats about the same diameter as a pencil lead, maybe a bit thicker. Too thin and it tangles up and is harder to lay neatly. I usually take a bundle of 3 or 4 of these wet them in water let them become limp, remove excess water, dredge them through glue, remove xs glue and lay them on the bows back neatly, like stacking bricks, smooth them down with a wet finger or other smooth object.
I use TBIII so after 2-3 hours its dry. Than I add another layer. Often the sinew bundles will seperate as they dry, because they shrink. I find that by letting each course dry before adding another I am able to fill in these empty areas with the next layer. Good luck and take your time, aint nothing fast about making a sinew bow, only shooting it!