1. It is always difficult for a beginner to get both the booties in the same size. Do not get disappointed, there are lots of others in the same boat as you. The first step is to make sure you do both the soles together and of the same size. If you do both the booties side by side, u can avoid changes to a large extent before it's too late. If you are making it for your baby, it's a good idea to try it on before weaving in the ends. A slight variation in size may go unnoticed when worn as crochet projects are stretchy and will take the shape of the feet.
2. Work into the back loops only means instead of inserting your hook under both the loops of a stitch, insert it into one of the loops only ( back as seen in pic) which will leave one loop of each stitch free. It helps to turn your work , like to turn the sides up from the sole.
3. Sole is worked in the round, or shall I say oval, around the foundation chain without turning your work at all. You work into all of the foundation chains and then go around to work in the back of the same foundation chain again. Making the sole involves a lot of counting. What you have to keep in mind is that the sole is made of two identical halves , one left half and one right half . So whatever increases you make will be symmetrical on both sides as seen in pic below. If your counting goes wrong then your sole will look like its turning to one side or have an extra bulge on one side. So make sure to count your stitches after each round. Use your hook to count your stitches and not your fingers. For beginners increases means doing more than one half double crochet or single crochet etc. into the same stitch. There are cases where the pattern requires the sole to have a sharper curve on one of the sides, but that is rarely required for baby booties. Each round in the sole is made using a mix of half double crochet, single crochet etc. This is to make the sole wider at toe side and narrower at the ankle side by making the longer stitches on the toe side alone. So watch out for the changes in the stitches in each round, do not end up doing sc where your pattern calls for hdc or vice versa. I hope these tips help you in clearing some of the confusions involved in making baby booties.