First, decompile apk file with [apktool](https://apktool.org):

```
apktool d <file_name>.apk
```

Replace assets or edit *Manifest.xml*. [smali2java](https://github.com/AlexeySoshin/smali2java) can be of use for code modifications.

Recompile apk:

```
apktool b <file_name>
```

Alignment is necessary for modern android versions (I beleave, 30+ SDK):

```
zipalign -v -f -p 4 <file_name>/dist/<file_name>.apk aligned-<file_name>.apk
```

Create a signing key. Fields values do not matter. Especially for personal use.

```
keytool -genkey -v -keystore my-release-key.keystore -alias alias_name -keyalg RSA -keysize 2048 -validity 1000
```

Actually sign apk with it:

```
apksigner.jar sign -ks my-release-key.keystore --ks-key-alias alias_name aligned-<file_name>.apk
```

It will overwrite the provided apk with a signed one.

We're awesome!