Finding a trustworthy source for signapk.jar is the most critical step. Because it is an executable Java tool, malicious actors could theoretically inject code into modified versions. Therefore, you should avoid random file-hosting sites like Mediafire or Mega unless you trust the uploader implicitly.
For most users without a full build environment, the best option is to download a pre-built binary from a trusted developer community. signapk.jar download
apksigner sign --ks mykeystore.jks --out signed.apk unsigned.apk Finding a trustworthy source for signapk
git clone https://android.googlesource.com/platform/build cd build/tools/signapk/ # The signapk.jar is usually built from source, but prebuilt exists in: git clone https://android.googlesource.com/platform/prebuilts/sdk # Look under prebuilts/sdk/tools/ signapk.jar download
java -Xmx2048m -jar signapk.jar ...