I mix fuel/oil in a large glass apple juice bottle that holds about 1/2 gallon using a funnel..
I put a strip of masking tape up the side and poured in measured amounts of water to accurately marked the tape for 1/2 quart, 1 quart, 1-1/2 quarts,etc.. Find a small plastic measuring cup for the oil.
For example, I use a 32:1 mixture. Add an ounce of oil to the measuring cup and then pour a bit of gasoline in that cup and swish it around to mix it.. Then pour that into the jar. Then fill the jar with gasoline to the 1 quart mark and swish it around again.
(One ounce per quart is 4 ounces per gallon.. and that would give you a 32:1 oil mix ratio. )
Oil must be well mixed into the gasoline. Premix it well before pouring in the tank.
And everyone will tell you DO NOT use glass containers for gaoline since if it hits the concrete and breaks you might start a fire from the fumes before you can swab it all up.. If you have a plastic see-through container, it is infinately preferable to using a glass jar or bottle.