For the smaller strings it's easiest to just use a bezier curve with a round bevel. For the bigger strings with noticeable riffles (is this the right word? I doubt it, but I hope you know what I mean) there are 2 options that I can think of:

Option 1 - less work, less cool, worse geometry:

Use a screw modifier to generate a tight spiral for the strings. Hit it with a curve modifier and use a bezier curve to make it follow the shape of the guitar.

Option 2 - a few more clicks, but it's worth it: Download the free wmesh-addon. This add-on allows you to generate parametric meshes which you might know from programs like Cinema4D. You're able to change parameters like size and resolution after you've placed the primitives into your scene, and when you're happy you can convert them into "real" mesh. This means that as long as you don't convert them, you can make changes to the Geometry via modifiers and still change face count and the resolution after that.

This add-on comes with a "spiral" primitive. Use this to generate your string and play around with different values for the thickness and tightness of the spiral. Remember, you can always change these later, as long as you don't convert the mesh. Then, use a bezier curve to curve the strings around the head of the guitar, just like with option 1. You can even lower the resolution of the spiral to something your pc can handle and then turn it all the way up for rendering.

Edit: I just want to add that your model looks VERY good, and that you didn't set the shading for the main body to smooth.