For spiking your soils, spike with your standard so you know the mass of analyte you spike and the mass of the sample. Such that 0.1ml of 10ug/ml standard would be 1ug of analyte, spiked into soil of 30g would give you a soil concentration of 1ug/30g or 33.3ug/kg of soil. (values are examples, just use whatever your actual values are for the process)
When you do the analysis you subtract any concentration found in the sample from the concentration found in the spiked sample, then compare it to what the known value should be and you have percent recovery.
The difference in external standard and internal standard is whether or not you are comparing the concentration/area of the analyte to that of another spiked internal standard analyte. The links in the last post will explain it in detail.