Try this. Put tongue into position for l (English l), but block off any possibility for air flow, more completely than for normal l. Don't say l, but force air through mouth only, passed tongue tip, (none through nose) and keep blocking off air in mouth with tongue similar to position for l. Tongue will resonate like jack-hammer (voiceless). This isn't a voiceless trilled r, but it gets you used to forcing the trilling action, so you know you can force a trill anytime. Tongue is in slightly different position for r, but once you know how to trill reliably, it will be easier.