Do you have "open.btc"?
If so; on OpenLedger or bitshares.org/wallet go to EXCHANGE (top menu bar)
Then on the top right click "Find Markets"
Search "open.btc"
And then click the "bts" button just underneath where you typed "open.btc"
In the top left corner of this new screen, click OPEN.BTC <-> BTS. It should then switch to saying BTS <-> OPEN.BTC
Find the box on the page that says "buy BTS"
Click "balance" to autofill the order form with your entire OPEN.BTC balance
Then click "Lowest Ask" to autofill the price at which you are most likely to make a quick buy
Click "Buy" and wait a minute or two for your order to execute
Done
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In future, you may find it easier to use a "Bridge" service to get your btc into bts.
Navigate to "Deposit/Withdraw"
Choose "Bridge" (not gateway)
Under "Transfer Service" dropdown menu, select "BlockTrades"
Under "deposit" just below, select your input cryptocurrency. In this case BTC. Then select "BTS" as the output currency.
A BTC address will the be provided on the right hand side.
Send your BTC to this address, and then blocktrades (a good reputable service) will automatically send the corresponding amount of BTS to your account
You may want to send a small amount of bitcoin to this address to test that it works. It may take a few hours, as bitcoin confirmation times are being very slow recently
I hope that helps