Every conversion is one decision and one multiply (or divide): am I heading to a smaller unit or a bigger one? Get that, and the zeros take care of themselves.
The One Rule: Smaller Unit, Bigger Number
If you move to a smaller unit, you'll have more of them, so you multiply. Move to a bigger unit and you'll have fewer, so you divide. The step size up the capacity ladder is always × 1000: mL → L → kL.
A Worked One
How many mL in 2.5 L? Litres are bigger than millilitres, so going down to mL means more of them — multiply: 2.5 × 1000 = 2500 mL. And the other way: 3000 mL into litres? Going up, so divide: 3000 ÷ 1000 = 3 L.
Choosing the Unit That Fits
The second job is taste, not arithmetic: pick the unit that keeps the number friendly. A cup of milk is about 250 mL — saying it's 0.00025 kL is technically right but useless. A backyard pool is best in kL or m³, not 50 000 000 mL. Aim for a number you could read out loud without losing your place.