It depends on how accurate you need to be !
rough and ready way (which I use in practise) - one drop from a dropper bottle is about 50 microlitres (uL), so you get 20 drops to a millilitre (mL). A teaspoon is about 5 mL so one drop in a teaspoon of carrier oil is 1% solution (5 x 20 drops in a teaspoon = 100, 1 drop in 100 = 1%)
to be more a precise you either need a very accurate chemical balance and do the following: weigh empty blending bottle and zero (or 'tare') the balance, add carrier oil and weigh, add essential oil and weigh again.
Calculate weight of essential oil by subtraction and then % blend = weight of essential oil / weight of carrier oil x 100
or some accurate chemical pipettes and measuring cylinders and do a similar calculation based on volumes.
If this sounds wrong to anyone please correct, it's a bit early for my brain to be doing maths