I'm not sure, I never used one before. But I think you install it with the piston at BDC (or at least where the piston surface is below the trapped compression ratio volume) and you turn the flywheel to TBC while reading the gauge. When the needle (I think it is analog) hits the highest pressure reading just before the pressure pushes the piston back down, that its the pressure you record.

If you can hold the piston at TDC with the trapped air in the cylinder, I guess you can also check for compression leak (i.e. the needle begins to drop while the piston remains held at TDC).