When do I switch to one nap?

Usually around 15-18 months on average, your one year old will go to one nap. Some hit this earlier and some hit this a tad later, which is totally fine.

When will I know we need to move to one nap? 

Here are some signs of needing to go to one nap. If you are close to the 15-18 month mark, and your baby may take two short catnaps or if your baby was taking two long (1.5-2 hour) naps and now taking one short nap and one long one, and falls between the 15-18 months (or close to this age). If your baby is doing this, then its time or it's getting close to transitioning into one nap. Other signs, is if your baby is fighting going into nap time or not falling asleep and staying awake for 30-45 mins, first move the nap back. These signs are just letting you know you are getting closer to moving to one nap. Sometimes, your baby may nap easily at first nap and not the second nap. Then just move the 2nd nap further back, giving more awake time. As soon as the 2nd nap starts to interfere with not going down easily for bedtime or fighting bedtime because he may not be that tired, then chances are, you need to move the am nap further back, and move towards going to one nap.

My baby is ready to go to one nap, but how do I do it?

My recommendation is keep your baby up till about 1245 pm. Give lunch around 11/1130 pm and then another snack around 12/1215 pm. Offer water, toys, whatever you can do to keep baby awake. If you know your baby can't make it to 1245 pm, then go somewhere in the car around 10am and let baby fall asleep in the car for a 10-20 min (no longer) catnap. Then keep baby up till 1/130 pm for a solid one nap. This way, you get a quick power nap, once you arrive to destination, wake the baby up, so you will have a great afternoon nap. You may need to do this for a few weeks or so, or a few times a week, getting a power catnap around 10am, and a longer nap in the afternoon if baby can't stay awake till 1245pm. Give yourself grace and your baby grace over the next month to transition into one nap. Some days may be a catnap and a long later nap in the afternoon, or a solid 1.5-3 hr nap. Just remember to give time for this transition, for mom and baby to get used to.