Until there was an interface to visualize and otherwise interpret the EOSCrowdsale windows, this was not an easy task. @lukestokes posted an interface that doesn't rely on MetaMask and I then forked and improved it. Having this interface made contributing to future windows for non-MetaMask users feasible, so here it is.

Important note If you have MetaMask, and have already contributed to the EOSCrowdsale through their interface, you can use the official EOSCrowdsale interface to contribute to future windows by selecting your desired window from the dropdown.

These instructions are specifically for contributing to a future window, if you are not familiar with this ICO and MEW, you should read EOS Token Sale with MyEtherWallet - The Complete Guide first.

Instructions

1. Read this article and follow the instructions
2. From the interface referenced in the last step, choose a window in the future you would like to contribute to. Note: Closed windows will appear red, current window is yellow, future windows are after the yellow row. You just need the number, for example 340
3. Go to https://www.myetherwallet.com/#contracts
4. Under heading Select Existing Contract in a dropdown, select EOS Contribution
5. Click Access
6. Under the "Read / Write Contract" heading, in the dropdown select buyWithLimit.
7. In the field day put the number you decided on earlier that represents the future window you would like to contribute to.
8. In the field limit put 0 (if you would like to know what this does, please scroll down.)
9. Load your wallet using any one of the methods listed.
10. Click Unlock button
11. Click Write button
12. in Amount to send to enter the amount you would like to contribute and allow the wallet to suggest a Gas Limit. If it does not autopopulate, enter 90000 or more. If your TX fails, increase Gas Limit. If you would like your TX to go through more quickly, adjust your gas limit accordingly.
13. Click Generate Transaction button
14. Check the details of the transaction, and if you are confident with the details you have entered, click Yes, I am sure! Make transaction
15. If all went well, a green bar will appear at the bottom of the page including a link to your transaction on the blockchain.

If you would like you can double check that this worked, after you have given your buyWithLimit transaction enough time to be confirmed, that is.

Notes

• If you contribute to a closed (invalid) window it will fail and gas will be exhausted.

About the limit functionality

When any positive number other than 0 is entered, it will protect you from contributing multiple times to the same window with the same amount. This is to protect you in the case of network congestion, and pending transactions. For most people 0 is adequate.

If you need more information about contributing to the EOSCrowdsale using MyEtherWallet, you should also check out EOS Token Sale with MyEtherWallet - The Complete Guide