Originally Posted by a7m

You will arrive to the platform which is outside (but right next to) the railway terminal (Belorussky Vokzal). The subway is easy to find: it is by the terminal's front entrance. Here is the google snapshot of the entrance (the metro entrance, marked M, is on the left).

https://maps.google.com/maps?q=belor...268.9,,0,-4.51

You can also look at the aeroexpress map:

http://www.aeroexpress.ru/en/sheremetyevo/scheme.html