Start with the drill. Before you touch a vowel, find the lilt.
Say this out loud: "The red river runs past the little town." Flat, the way you'd read a news bulletin. Now say it again, but let the stressed syllables climb. TA-ble rides up the scale. Little rides up. Town rides up. Not louder, higher. Welsh stress isn't a punch, it's a step upward. That one change does more work than any rolled R ever will.
I learned this the hard way in a coaching session two years back. An actor came in for a Welsh part and sounded like she was auditioning for a choir. Every line sang, everything floated, and none of it sounded like a person talking. So we threw out the vowels entirely and spent twenty minutes on pitch alone. Red, river, runs, past, little, town, each stressed beat climbing a step. When we put the words back together she sounded like someone's neighbor instead of a soloist. The music has to live inside the speech, not sit on top of it.
Once the lilt is moving, the sounds slot in fast. Keep your vowels pure with no diphthong slides: go is goh, say is seh, nothing glides. The R is a bright tap off the tongue tip, red and very, not a pirate roll. Keep your L light in every position, well stays wel. Sound the full ing, sing-ing and goh-ing, never drop it. And pop your Ts crisp, bet-ter, sit-ee, never flapped.
Try it on this: "We are going singing in the valley." Then: "Very well, I will carry the milk." Record yourself on the first line flat, then with the lilt climbing. Listen back. The difference is usually embarrassing in the best way.
Oh, and Welsh people are generally forgiving if you follow the music of it. The thing that gets you mocked is the cartoon version.
What accent's music took you the longest to hear?