Hi Cara, you can start anywhere you want to, but I usually start just past a corner. Just attach your yarn by making a slip stitch at the place you choose to begin and then start crocheting your border. I like to do a round of single crochet first and then the border, but crochet it how you like and it will be lovely.
You can refer to the link I provided above to see how I crochet a border around a knitted blanket. I also have other video tutorials on my website you might find helpful.