Let's start at the beginning. You have enough yarn, although I would prefer smaller needles than U.S. 19. You need a gauge swatch to determine what YOU think about the needles.

Cast on 30 stitches and knit 30 rows in the pattern you are going to use with the two strands of yarn and the size 19 needles.

Now: Evaluate the swatch. Do you like the fabric you made? If not, then change to a different pattern, smaller needles, more strands of yarn, or whatever you think would improve the result. Pull out the yarn from the swatch you made and make another swatch. Repeat swatching until you have one you like.

Next: With the approved swatch, measure a four-inch square out of the middle. Count the number of stitches and the number of rows in the four-inch square. Divide by 4 to get the number of stitches per inch to the closest fraction. Multiply the stitches per inch (including the fraction) by 32. That's how many stitches to cast on. The number of rows isn't critical for a baby blanket--you can just knit until you have 32".

Or when you like the fabric you get in the sample, you can knit the blanket on the diagonal until the two sides measure 32". Here's how to knit a baby blanket on the diagonal:

Cast on 3 stitches.
Row 1: K3
Row 2: K1, YO, K1, YO, K1
Row 3: K5
Row 4: K1, YO, K3, YO, K1
Row 5: K7
Row 6: K1, YO, K5, YO, K1
Continue in this manner, knitting each stitch on the odd-numbered rows and increasing with a YO at each end of the even-numbered rows until each side measures 32". Then decrease as follows:
Next even numbered row: K2tog, YO, K2tog, K to last 4 sts, K2tog, YO, K2tog.
Next odd numbered row: K across
Repeat until you have 3 sts; bind off.
Run ribbon of the appropriate length and width through the eyelets on each side of the blanket.

Addendum: I have some of the yarn you're using, so I made a swatch. I used two strands of the yarn to cast on 20 stitches on size 19 needles and knit an inch, then changed to size 17 and knit an inch, then changed to size 15 and knit an inch, then changed to size 13 and knit an inch. I liked the fabric at size 13, so I knit another inch and quit. The gauge I was getting was just under 3 stitches per inch (2.9, to be exact). So if I were to be making a blanket in Caron Simply Soft Brites using garter stitch, 32 inches wide, I'd cast on 94 stitches to size 13 bamboo needles. You would use your own gauge, of course, and adjust the cast-on for any requirements of a stitch pattern you are using (such as multiples for a pattern repeat).