Details for halfwave dipole for an 868MHz band are quite simple:
- two 0.8mm copper wires (used for welding) cut to 77mm and solder to the end of coax cable. Here I've used RG174
- put that thing into a 3D printed casing
- put antenna tubes around wires. I got mine from here: https://goo.gl/HrWNcY
- glue everything together
- you are done!