Mix 1:1
Tablespoon of melted buter for each cup of breadcrumbs
Toss together so they're well mositened (think of something just a tad less buttery than making graham cracker crust or streusel topping).
The 1:1 ratio is actually just a starting point, you may need to tinker with it. You can keep leftover b uttered breadcrumbs (in airtight container) in the fridge for about 2 weeks, or freeze em for making casseroles later.