You need Google Chrome (the app should support soon other browsers) + don't forget to set the default input in your OS preferences. If you plugged a guitar in, then press the pulsing button in the middle of the Amp to activate it. Otherwise you can try the amp with the audio player on the left, press play and use the preset menu or the buttons/sliders.
Signal goes first throught hicut and low cut filters, then through 4 filters (F and Q), then to a distorsion stage (Ks), then to a tonestack (B/M/T/P) then to a master volume. It is then post processed by the EQ and a reverb, for fine tuning the tone, set the EQ after the rest. Finally there is a cabinet simulator (bottom of advanced settings).
Check this to understand how filters work.
Press play to preview presets
Choose a demo sample:
Low and high cut frequencies
Cut off frequencies (low, mid1, mid2, high)
Presence (Q3, Q4), resonnance (Q1, Q2) associated to filters above...