I did a lot of provisional casting on and picking up stitches. And I highly dislike sewing, so I just connect everything together as I knit. Here's how...
(note: the boxes are blocks of knitting. Lines are ridges of garter stitch. For ease, I will refer to a block of knitting that is already there as black, and the block that is being worked red.)
Scenario 1: Attach along side ~ When you get to the end of every other row (where the end is next to the other block of knitting), slip your needle into a hole along the side of the black block. Then knit both the black stitch and the first red stitch together. This creates a little wrap-around of red around black, but since you're doing it regularly up the side, it doesn't look messy.
Scenario 2: Bind off against another block ~ I guess you could use the kitchener stitch. But this method produces a line of the black's edge and a red line from the bind off. Take a crochet hook and, one by one, take each red stitch and pull it through a hole in the black, replacing the stitch on another needle. When all the red stitches are pulled up through the black, cast off.
I made mine with all different sizes of knitting. I like the crazy look of it--my sole aim is to not have two blocks of the same color touching.
And because it's scrap yarn, there's a LOT of joining yarn. Which is another one of those things I dislike (but not as much as sewing). ANYWAY, I tied knots. Lots of 'em. Let me repeat that. THERE ARE LOTS OF KNOTS IN MY KNITTING! (Ooh, rhyme) Yes. For some reason, every site I've ever looked at has denounced knots. Heresy! they say. Abomination to the knitting! But knots make joining yarn easy (and I feel very secure). They take up less yarn. They are only ugly if you look at the wrong side of the work. And the thing's not intended to be worn, so comfort's not an issue (actually, I'm not really sure what it is intended for...hmm...). So how I join yarn is, I just stop with the old strand and start with the new (making sure the ends are on the wrong side of the work). After I work a few more stitches with the new strand, I take the dangly ends and KNOT THEM TOGETHER!!! I guess I'll weave in the ends. Maybe. Or leave them dangling. Eh. Or... KNIT THEM INTO THE WORK! (Of course, make sure to leave the itty bitty ends hanging out the wrong side) If, like me, you're not extremely picky. After all, that's what the double-stranding technique is when you weave in the ends.