Exchanging BCH to BTC

You can exchange your BCH for BTC and multiply your wealth by 1.075 (at the time of writing).

Send your BTC to a new key/wallet.
Download and boot Tails
mkdir -p /home/amnesia/.electrum/wallets
Download, unpack and run http://www.electroncash.org/
- 10b5e65608e452a174d28b5030796d6a712e8cb7 Electron-Cash-2.9.2.tar.gz
- 8cb6a0840ad25b992a6703970423343705c2e03689bbb14fb6b893f313e7eb8b Electron-Cash-2.9.2.tar.gz
Restore your old wallet from key/seed (i.e., the wallet that has now zero BTC balance, as you have sent out the coins in the first step) to Electron.
Create account at https://hitbtc.com/ (e-mail verification needed, but no name, ID card etc.)
Click “Account → BCC → Fund”, send coins from Electron to the generated address.
Wait 2 blocks.
Click “Transfer Main → Trading”.
On “Exchange” tab select “Instruments” box and select BCC
In “Sell BCC” box on “Market” tab click on “Balance” and then the big red button.
Wait a few seconds.
Now you have BTC.
On “Accounts” tab select “Transfer Trading → Main”
Select Withdraw, enter your BTC address. Wait a few blocks.

Exchanging BTG

This was not tested.
WS charges 10 % processing fees. Unless you are a Bitcoin millionaire, paying 10 % is probably worth the convenience.
WS claim they support multisig wallets too.

Send your BTC to a new key/wallet.
Paste your private key to https://dig.walleting.services/ (see, this is why it is very important to move your real Bitcoins to a different key beforehand)
Send your BTG to HitBTC and exchange them as described above.