When modding with heads and bodies that do not initially belong to each other, we often run into some problems. One of them is purely visual and generally just referred to as ''neck seam''. While this tutorial covers the removal of the mesh side of neck seams, it can also be applied to any other sort of mesh seams you want to get rid of.

1. Open both of your models in Blender and scale the head how you want it on the model. Depending on the models used yours scene will look like this:

2. Go into Edit Mode on the head model and select the portion of the mesh that is mostly covered by the body mesh. Do this as clean and symmetric as you can.

After doing this press x and then Faces.

3. Select all of the remaining head mesh and and remove doubles with a value of 0.000. After this, press Ctrl+n to recalculate Normals. Repeat this step on the body mesh aswell. 

4. Switch to Object Mode and select both meshes, then join them with Ctrl+j.

5. Return to Edit Mode. You will now have to weld vertices one by one. While this may sound pretty time consuming and annoying, it is actually very easy and won't take a long time. Your models' polycounts may differ, do not care about it too much as of yet.

Select two vertices that are closest together like shown here:

6. Press Alt+m to merge those two vertices into one. Blender provides you with a couple of options. I often choose 'Center' but you can choose the one you like the best:

7. Not every vertex will have a counter part on the other mesh. Ignore those for now and join the ones that are closest to each other.

8. Keep doing this until your mesh looks something like this:

9. The problematic areas look like this. There is a vertex on the body mesh which does not have a counterpart on the head mesh. 

10. Select the upper face seen in the circled area. It's the one face that we have to turn into two in order for another vertex being created. 
After selecting just that one face go to Mesh->Edges->Knife Subdivide. Alternatively press Shift+K.

11. Out of these options, select 'Exact Line'.

12. You will see a small knife icon (it is not visible on screenshot). Click once at 1 and again at the position of 2. Basically up and low of your selected face. To confirm press Enter.

13. Repeat this step on every face that still needs a vertex counterpart for another vertex. Once you are done your mesh should not have any seam anymore:

14. All that's left to do now is to seperate the head and body mesh again so the materials will not mess up in XPS. In Edit Mode, select all and press p. Choose 'By Material', this will seperate it according to the assigned Materials it had before.

15. Select the head mesh and in the lower toolbars select the Material of the body (usually this has the name of the body mesh). Delete it so only the head material is left.

Repeat this step on the body mesh and delete the head material from it.

This is the end of the tutorial! Congratulations if you made it to this point!  \(^~^)/