You have done the job of achieving good textured milk and good tasting coffee which is most important.
First of all, latte art is very much achieveable using a 300ml jug but i believe better results can come from 600 ml jugs.
If you can get one 600 ml jug, steam in the 300 ml jug then pour into the side of a warmed 600ml jug with enough milk plus a bit more to fill the cup.
If the jug is too full, and you pour, you can't get the tip of the jug close enough to the surface and the milk ends up going in through the crema and washing out.
So, brew a well extracted espresso into your cup.
Start with enough milk in the jug to fill the cup.
Tilt the cup at an angle towards the jug so that the tip of the jug has easier access closer to the surface.
Pour from a height with a fairly thin steady stream to pierce the crema. You can pour directly in the middle or swirl it around until the milk comes halfway or 2/3 up the cup.
Bring the tip as close as possible to the surface of the crema and start pouring a small bulb to start holding up the microfoam and prevent it from washing out.
Pour your desired art. If you're starting out, I recommend the heart first as this is the easiest to achieve and will give you more confidence to do other art.