No matter what you want to do with a router, you need some way to control and guide it. This is true even with free hand lettering. Notice in the first picture how I anchor my hands down to the wood so that either one of them is almost stationary while the other moves, or both are dragging along on the wood to prevent the router from taking off in some strange direction. I hate using those D handle routers for free hand work, I always want my hands down on the wood.
The guide that usually comes with the router is a simple side guide. You can do a lot of good work parallel to the edge once you master the tricks of this one. One little trick. Trying to cut a slot or design in the exact centre of a board? Cut close to the centre with a bit smaller than you need, and then repeat the cut from the opposite side. The slot will be larger than the bit, but perfectly centred because the cut are the same distance from each face.
Most edge work is done with the simplicity of bits that are guided by a ball bearing built right into the bit. The trick here is to be sure that there is not glue or sap causing the ball bearing to seize up, as then it would not walk along the edge but spin and burn the edge.
Many people like to do most of their work with the router stabilised under a special table and you move the wood through the bit. All the same guiding tips work here as well, just watch out because your fingers are no longer safely on the router handles.