First. Bribes.
Offer double the existing guns worth as a pass through deductible to turn them in. Ammo too, buy it back.
Second, background checks and 90 day wait period to conduct those tests properly and flag questionable applicants. This also works as a very good "cool down" period
Third, national database of gun sales and flags from other states
Fourth, kill the gun shows or make it as onerous to purchase one there
Five, all private gun sales must occur and be witnessed at a licensed gun shop and entered in the national database
Six twenty one years old to purchase. If alcohol kills and they made it 21... do the math
Seven continuing education courses every two years to maintain your license.
Eight - that license. Everyone should require one.
That's just the start