1. Measure blood concentration over time through IV administration
First you need to administer a drug in a manner that has 100% bioavailability, or completely enters the systemic circulation. This is achieved through intravenous (IV) administration of the drug. One then measures the plasma concentration of the drug over time to derive what's called Area Under the Curve (AUC).
2. Measure blood concentration over time through different route of administration
Next, you administer the drug, but through a different route of administration e.g. orally, sublingually, rectally, intramuscular... whatever. Like before, you measure the plasma concentration of the drug over time to derive another AUC. You then compare the AUC for this route of administration and the AUC for when it has 100% bioavailability. The ratio of the AUC relative to the AUC with 100% bioavailability will allow you to calculate the bioavailability of the route in question.
Although effective orally, methyltestosterone is more effective by these parenteral routes, which are said to approximately double its bioavailability.
These are the AUCs for 10 mg sublingual (Left) versus 25 mg oral (Right) methyltestosterone:
Essentially, it's stating that for the same dose, sublingual administration will result in roughly twice the AUC of oral administration.
From: Alkalay et al.(1973). Sublingual and Oral Administration of Methyltestosterone. A Comparison of Drug Bioavailability. Journal Of Clinical Pharmacology And New Drugs, 13, 142-151. https://doi.org/10.1002/j.1552-4604.1973.tb00075.x