Catholics are required to fast on Ash Wednesday and Good Friday. Adding another day of fasting is a way of practicing an additional sacrifice during Lent. You can decide to fast one day per week. (Friday would be a traditional choice for this.) Or you can just fast a single day or a few days of your choosing.
Traditionally a fast means two small meals and one regular meal per day, with no snacking between meals. But since this is your own additional fast you can choose how to do it. Just decide your parameters and stick to them. Fasting is an act of learning self discipline, so make it a bit of a challenge, but not so difficult that you are setting yourself up for failure.
You can also choose whether to abstain from meat or not on your voluntary fast day. I recommend it, but you also have to consider those around you. For example, as a member of a family, you can’t expect the family cook to prepare a special meal for you. So if abstaining is going to cause more problems than do good, you can make your own choice since this is your own personal act of sacrifice, outside of the required abstinence. However, if you choose to do your fast on Fridays during Lent, the abstinence will already be “built in”, making Fridays a logical choice for this activity.
And remember that fasting is not dieting. The purpose of fasting is to grow closer to our Lord and to empty ourselves of our attachment to worldly things. So take some time to think about the meaning of love and sacrifice.