How do I add blocking to floor truss joists?
Along the wall at the top of your photo, simply cut blocks that fit snugly between the trusses and toenail them in place. 

In the field, float blocks under the edge of the remaining subfloor and screw them through the subfloor. Toenail them once that's done. 

At the right wall, lay a flat 2x4 atop the stiffback and under the subfloor for at least 4-6". Screw and glue it to both. 

At the left wall you have more of a challenge. In hindsight you might've left a fringe of subfloor along the wall to reconnect to. As it stands, you'll have to block up from the trusses somehow. I can't see what's behind the insulation and wall, so it's difficult to be specific. 

The critical factor here is that the subfloor panels are connected at the seams. It's not as critical that the blocks are attached to the trusses where they're perpendicular, though that's not a bad thing. Along the wall it's more important that they be connected to the joists because you can't connect them to the adjacent sheet. 

Install all sheets using construction adhesive and ring-shank nails or screws and you'll have a very solid subfloor.