ok quit easy for example the successor of Truecrypt is Veracrypt - its a very crucial part that your encryption programm is indeed untouched by any third party. If its 100% opensource its no big deal to build your own exe or container like deb in linux.
Veracrypt uses only a PGP.signature.I assume that you know what PGP is an how it works.
First of all you need to post on the webpage the ID & Fingerprint of your public Key. Thats very crucial because a third party could intercept the download of your public key.
After that the user can verify that your public key is indeed the right one, if the public key is well known other persons/parties can also sign it to push the trustworthy of it.
The last step is that the user downloads the signature file and verifies the integrity of the file = good or bad signature!
A bonus is, if you put a integrity check on the page where now the old MD5 stands an write there: SHA256 and the the checksum. For the fast ones
Because PGP isnt well known, i would recommend that you put a link to the signature file like the Tor-guys use "Whats a signature - how do i check it" -- "https://www.torproject.org/docs/verifying-signatures.html.en"
Hopefully it helped !