Learning to isolate your tongue from your jaw and soft palate takes time and practice. There is no quick fix for this one, I'm afraid. You just have to work, or play, at it until your tongue and jaw learn how to play independently. Of course, this takes time. But an effective exercise is one that targets the process involved effectively, and sometimes doing a familiar thing in an unfamiliar way can make enough of a difference that what you learn from doing it crosses over to doing the familiar thing back in its familiar place.
The movement of the tongue is very varied. You can bunch your tongue up like a ball, fold it, flip it, cup it or arch it. The action we're looking at today can be described as "flapping." You want the front edge of your tongue to go up the the area behind your upper front teeth, as if it we a flap, hinged at the centre of your tongue. The front edge goes up, the front edge comes down. Now, to do this, your tongue actually is "curling" back on itself slightly, but the image of a flap valve (like you'd see in the bottom of a toilet tank) could be helpful. For people with very short frenums on the underside of the front of the tongue (aka ankyloglossia) this may be very hard to do.
To exaggerate this action, we're going to move the tongue "out of the mouth," that is, do the flap action with tongue stretched forward, out of the mouth. Begin by sliding your tongue forward onto your lower lip. Fro there, bring the front edge of your tongue up to cover your upper lip. Finally, relax your tongue back down over the lower lip, and then let it slide S L O W L Y into your mouth. Do the combined action slowly, maybe as much as ten times in a row. As you can see from the animated .gif file that accompanies this step, your jaw is likely to close as your bring your tongue up to the upper lip. This is partly due to the length of your tongue. When we move the action back inside the mouth, we'll try to keep that jaw relaxed. The next step is to add sound: sigh out on "ah" on the tongue sliping out portion, make an "n" sound as your touch your upper lip with the front edge of your tongue, and then open back up to "ah". After a few of those, narrow your tongue slightly, so that your make a "L" sound when your lift the tongue up to your upper lip.
You now want to do the whole thing with your tongue inside your mouth. Start by dropping your jaw, and then bring your tongue up to behind your upper front teeth. It's not very far, compared to what you were doing with your tongue out! Start with a very simple /n/ sound, and then go to the narrower tongue action of /l/.
You can see a video of the tongue action here.
Next Step: Soft Palate Floating