there's no "best" method, in terms of how you build your models for engines like hl1.
box, poly by poly, it's all the same. what's important to learn, is how and where to place your edges and verts. where you need a dense mesh and where you don't. where you can fake detail with a texture instead of modeling it all on.
bear in mind, too, that half life had harsher limitations than current gen engines. and while the theory remains the same now, you will be pressed more on hl1. make sure every single vert is defining a shape, that every edge will help the mesh deform better when animated.