Keep the thinnest textures close to your body. Bulkier items should be worn on the outer layers and chosen sparingly so as to keep the silhouette sleek. The outer layer should always be loose too, so your mobility isn't restricted, if opting for an overcoat, you want one with raglan sleeves, to avoid a bulky build up at the shoulders.
Layering, of ofcourse isn't exclusive to smart wear either. Casual wear makes for a great base for layering. For instance, start with an off white flannel shirt and a thin wool camel cardigan, tone them up with a fitted denim jacket and an oversized brown wool knee-length coat with skinny jeans and brown leather Chelsea boots for a casual smart look.
As for colors, start with neutrals and add a dash of color, one textured item or one bolder pattern into the lot. The key is to keep the whole outfit balanced. You can totally work with a monochrome palette. Put together an all grey outfit with a turtleneck and a gilet, to play on different sleeve lengths, and grey fitted sweatpants. Throw in a leather jacket, a grey beanie hat, maybe some golden signet rings and you have got yourself a casual, careless style. You also have the opportunity to go all out with a completely oversized outfit to show off your carefreeness; in that case, brands like Yohji Yamamoto and Fear Of God are your best friend.