1. Take the base APK and decompile it
2. Take all other split APKs and decompile them
3. Now put all files of the split APKs to the base APK but do not override files (there will be files already existing in the base APK, do not overwrite them!)
4. Open the AndroidManifest.xml in the decompiled base APK and remove this setup: android:isSplitRequired="true"
5. Open the apktool.yml and add in the doNotCompress tag of the base.apk everything you have in the other split APKs
6. Now you have to do the annoying job to check the .xml files in the APK/res/value folder in all the split APKs and add whats missing inside that files from the other split APKs to the base APK .xml files.
-> NOTE: This step is time consuming but it can be skipped on some games. Try first if the game works without this step to save time. If it works, skip it.
7. Compile the base APK, sign it or make it unsigned with the META-INF of the base APK.
8. Done =)