The most important first step is to test your current levels. Then you can use a Vitamin D calculator[1] to help get you a starting point for how much to supplement based on your desired target level -- recommended target range is 40-60 ng/ml (100-150 nmol/L). Then re-test in 3-6 months to check levels and adjust supplementation as needed.
[1] Vitamin D calculator: https://www.grassrootshealth.net/project/dcalculator