The best option is to go on the Ekamai bus, whichever station it leaves from, and get off at the first skytrain station ( the slow grind along Sukhumvit isn't worth the few baht savings to go all the way to Ekamai ). However, you can't get stowed luggage out if you do that, so carry on bag only.
Skytrain all the way to MBK with a change at Siam.
If time is important get to bus station early enough to get the bus you need, unless one can book ahead now.