A Quick guide to using the BTS Skytrain from a regular Joe traveller i.e I don't live in BKK but do come here often.
There are 2 BTS lines. 1 in dark green and other in light green. The process works the same. This example is on the light green line for ozebeachbabe example.
If staying at HIX Siam (which seems to be closer to National Stadium vs Siam stop)
You will see a map like this which shows all the stops
Each stop you are at will have this same graphic next to the ticket machines except the numbers (fare pricing) will change subject to where you want to go.
Just look at the station you want to go to (regardless what line it is on). There will be a number. This if your fare price for a single 1 way ticket.
Not the direction of the train you will need to catch as this is the platform / train direction you will want.
Turn to the ticket machine. Press the button with the fare price (from the graphic on the pic above).
Insert your coins and collect the ticket and your change. This machine will only accept coins THB 1, 5, 10s. Most don't seem to accept notes.
If you need change ... get it from your hotel prior OR go to the manned change booths at the station (they can also tell you the price you need to pay if you can't work out the graphic. They will give you change and likely send you back to the ticket machine)
Here you can see the manned changed booths as well as the turnstiles to enter the platforms
And there you have it. To go back to where you started just reverse the process.
Now ... to get to the Hilton ... from the dark green line. Catch Skytrain to Saphan Taksin.
Exit train and go to exit # 2 and go down stairs