Here's how to send Ethereum from an Idex account to a Bitstamp account.

Step 1: Get your Bitstamp Ethereum deposit address

Step 1a. Go to https://www.bitstamp.net/ and log in.

Step 1b. Once you're logged in, go to https://www.bitstamp.net/account/deposit/eth/ and copy the ethereum deposit address.

Step 2. Send the Ethereum to Bitstamp

Step 2a. Go to https://idex.market/balances. You will need to log in if you're not already logged in. You should see this page:

If your Ethereum is in your "Wallet Balance", then keep following these instructions. If your Ethereum is in your "IDEX Balance" however, then you will need to move it to your Wallet Balance first. You can find instructions on how to do this here.

Step 2b. Click 'transfer' on the right side of the page

Step 2c. Paste the Ethereum deposit address and click 'transfer entire balance'.

Step 2d. Change Gas to 40000, otherwise the transaction will probably fail.

Step 2e. Subtract roughly 0.001 from value in the "Amount to transfer" box. The reason you have to do this is to pay for miners' fees. If you don't do this, the transaction will also fail.

Step 2f. Click transfer.

Step 2g. A Confirm Transfer box should appear. Click CONFIRM.

Step 2h. A box that says "Preparing to send transaction" should appear. Let it disappear on its own. DO NOT click the OK button as doing so seems to make the next box not appear.

Step 2i. This box should appear. Click on the link in the box and it will take you to an etherscan.io page.

Step 2j. Wait a bit on the etherscan.io webpage for the transaction to go through. After a minute or two, you should see a success message:

However, if your transaction says "Warning! Error encountered during contract execution [Out of gas]" then go back to https://idex.market/balances, and try again, this time setting the Gas a little higher.

And if you see this error:

...then it means you have to try again but this time subtract more from the value in the "Amount to transfer" box.

Step 2k. Go to https://www.bitstamp.net/account/balance/. Your Ethereum should be there.