There are lots of tutorials on picking up stitches along the edges (and unless otherwise stated you should always pick up loops and knit into them) but sometimes you’ll need to pick them up from the middle of the knitted fabric. Toys, flip-top mittens and sweater plackets all use this technique. The stitches can be picked up in a row, a vertical column or even along a diagonal. Be careful when picking up across a row that all of the stitches are picked up from the same row.
Insert the needle into the middle of a stitch.
And bring it back out through the middle of the stitch above, picking up one strand.
Insert right needle and knit the picked up loop.
One stitch picked up and knit.
I find it easiest to pick up the work and hold it in my left hand so that it's folded and I can pick up stitch along the fold. If you can maintain the tension on the yarn try picking up the stitch and 'hooking' the yarn through with just one needle.
A row of picked up stitches.
In garter stitch insert the needle under the upper bump of a stitch.