With "R", touch your top teeth to the inside of you bottom lip. Try to not use your tongue, it ideally shouldn't be touching any part of your mouth. With "L" the other answers are right. Hold your tongue to the back of your top teeth.

In the "L" sound your tongue starts up against the back of your upper row of teeth. You can keep it there to hold the sound or release it to flow into the rest of the word. The"R" sound is a little different. Your tongue should pointed toward those top front teeth but it shouldn't touch them. That's a key difference!

Hey Fumiya. The "L" is a bit easier to explain. When you try to make the sound, touch and hold your tongue right behind your teeth then try and make the sound. Hopefully that helps!