Start small. Don't do a three-day or one-week fast your first time. You may choose to skip a meal. Remember, it is difficult, but you will live. Drink lots of water but not all at once. If you can't make it on just water you can fast on diluted fruit juice as needed. A water fast is preferable.
No cheating. The Lord has convicted me of drinking lots of water to feel full. You need LOTS of water but sip often. No gum, mints, breath spray, soda or flavored liquids or candy.
Also spend extra time on the Lord on fast days. Don't spend hours watching television for example or reading non Christian material. Limit distractions such as social media. This is not like every other day. Make it a special day to the Lord. Be effective, bring value and make it efficient and purposeful. Get the most out of it. Put extra focus on the Lord.
How long should I fast? Determine that with the Lord. Start small. Try fasting a meal or two. A one day, three day, one week, three weeks and 40 days are the most common, however.
How often? I currently set aside two days of the week. This is between you and the Lord. Let Him direct and guide you. Pray and seek the Lords guidance on this. God bless.