• Create a new ue4 project , set the project type to c++.
• If you already have a blueprint only project add a blank c++ class .

• Open the Project folder (where you can see the .uproject file).
• Create and open (or Open if it exists) the Plugins Folder.
• Right click and open git bash and run :
  git clone --recurse-submodules https://github.com/irajsb/UE4_Assimp/
• if you can't see the git bash option you need to install git from Git
• you can see if this step was done correctly if Plugins\UE4_Assimp\Source\ThirdParty\UE_AssimpLibrary\assimp folder is populated.

• Open the Plugins\UE4_Assimp\Source\ThirdParty\UE_AssimpLibrary\assimp\ Directory and open a terminal and run :
  cmake CMakeLists.txt -DLIBRARY_SUFFIX:STRING=
• Open the generated project files (.sln file in windows platform )
• Select Release build configuration x64 and build the soltuion

• open solution file in root folder of the project and rebuild the project .

Instead of commands at step 3 use cmake CMakeLists.txt -DLIBRARY_SUFFIX:STRING= -DCMAKE_OSX_ARCHITECTURES=x86_64 -DCMAKE_OSX_DEPLOYMENT_TARGET=10.15

Make sure you have ninja installed and added to path

Make a folder named build inside assimp then run
cmake -G "Ninja" -DLIBRARY_SUFFIX:STRING=" " -DCMAKE_BUILD_TYPE=Release -DASSIMP_ANDROID_JNIIOSYSTEM=ON -DCMAKE_TOOLCHAIN_FILE="$env:ANDROID_NDK_HOME\build\cmake\android.toolchain.cmake" -DANDROID_NDK="$env:ANDROID_NDK_HOME" -DANDROID_ABI=arm64-v8a -DBUILD_SHARED_LIBS=1 ..

and then

ninja