Try this -
Step 1: slowly say the normal L sound preceded by all the vowel sounds you can think of -
e.g. AL, ALL, ELL, ILL, OL, OOL, ULL....
and pay attention to where your tongue is position everytime you make the L sound.
Now, repeat all those but this time, instead of making the L sound, keep your tongue exactly where it is (i.e. about to make the L sound), but just blow, letting the air go either side of your tongue.
Keep doing this with these one syllable combinations eventually getting faster, then when you feel you've got the hang of it, practice it in actual Welsh words where the LL comes in the middle or the end, again saying them slowly to begin with then speeding up.
You'll soon be able to do it without even thinking about it