You can get streamers at a party store. For a typical .40 size trainer, you can use the full 2" wide party streamers easily.

To attach them, tie a length of string to the streamer, then tie it around part of the plane. I usually loop it around the entire tail group and lead the string back so that it doesn't tangle with the control horns. It's not critical though, and it's really rare for a streamer to cause problems.

You'll want to use a fairly long string, probably 10 foot or so, as that lessens the chance of a mid-air, since a mid-air with trainers is going to make a mess.

As for best way to get him, that's going to depend on how you and he fly. If you're both low on flying hours anyway, you probably aren't going to do well at tactics beyond the Nelson approach ("Forget tactics, go straight at 'em"). Unfortuantly, this often results in mid-airs if your opponet is taking the same approach. You should look to fly around the other guy to get behind him. Don't fly directly towards him until you're behind, if heading directly at him, dive or climb and a slight turn, then reverse the turn to swing around behind him. Once behind him, wait for him to turn, then cut inside the turn and pass through the streamer as you go from inside the turn to the outside.