This is a very broad topic unfortunately, and there is no single way, simply because there are so many good ones. It will depend on what kind of terrain you're making. Will it be flat only with levels? Will it be hilly? Is this going to be an overhead view where only part of the terrain will ever be visible, or will it be a FPS type of view where you can see forever into the distance? Also, how are you managing your terrain? This is important, because you may be using LOD which would limit some algorithms while opening up new prospects for other algorithms. So, if you provide some good detailed information on what your terrain can do, then we can give a more direct answer.

First off, don't apply a single texture over the entire terrain unless you will be adding details on top of it. In other words, that will work for far-off places in a FPS style, but you'll want to keep details around the player.

AP is correct in that you could go about it the easy way and add textures that are used to blend between grass and rock for instance. You could get yourself into a pretty big mess if you aren't careful with that though.

Look into texture splatting, multi-texturing, vertex shader terrain texturing, and any combo thereof. Also take a look through the Articles link at the top of this site. There are some articles specifically for terrains. If you aren't familiar with LOD and frustrum culling algorithms, you'll want to look into geomipmapping, ROAM, quadtree (or octree), etc. All of this will give you some ideas to work with and you should have a firm understanding of their advantages/disadvantages and how they impact the terrain texturing. You'll be doing good if you spend a week or two understanding all of this (few hours a day I mean).

Chris