How do I decide how much to put in each sinking fund every month?

A sinking fund saves gradually for a known future expense so it never becomes an emergency. To size each one, estimate the total cost and divide by the number of months until you need it. For a $1,200 annual insurance premium due in 12 months, save $100 a month; for a $3,000 vacation nine months out, save about $334. List every predictable irregular expense, car maintenance, holidays, property taxes, annual subscriptions, then total the monthly contributions to make sure the combined amount fits your budget. If it does not, stretch timelines or trim goals. Keep sinking funds separate from your emergency fund so you never confuse planned spending with true emergencies. Many people keep them as nicknamed sub-accounts in a high-yield savings account.