After you have logged in to your MyEtherWallet account, click on the "Send Ether & Tokens" tab.
Next, find the payment instructions detail on the ICO token sale page, for example like the one below:
After that, I fill in the "Gas Limit" box because I usually get an error message if I fill in the payment address and the amount first. Make sure you put the "Gas Limit" as stated in the payment instructions and not the default pre-filled "Gas Limit." You can copy and paste it.
Next, I fill in the "Data" box in advance to avoid me of being forget to fill it. Click on the"+Advanced: Add Data" to reveal the "Data" box. You can copy and paste it too.
And finally, you can fill in the "To Address" box and enter the "Amount to Send" box. You can copy and paste the payment address.
If all have been filled in, click on the "Generate Transaction" button bar, and a " Send Transaction" button bar will appear. Click on the "Send Transaction" button bar.
if everything goes through, this message will appear at the bottom of the page.
Hope this helps!