You need the voting system to be end-to-end verifiable. The secrecy of the vote must be preserved, meaning that not even the servers can know, at any step of t

You need the voting system to be end-to-end verifiable. The secrecy of the vote must be preserved, meaning that not even the servers can know, at any step of the process, that voter A voted for option X. Voters must be able to verify that when they choose option A, option A is correctly coded into their ballot. Voters must be able to verify that their ballot is included in the tally and that their ballot is unmodified. Anyone must be able to verify the process of mixing/anonymizing, decrypting and tallying the ballots. So it's not easy at all because the requirements are quite contradictory. But you can do it with mixnets, for example, using them in a similar fashion as Tor. That's the way Scytl or nVotes do it :)