Installing a US trigger group is easier than an original trigger and gives you 3 US parts. US trigger groups also shoot better in most people opinion. Once your receiver holes are correct, installing the trigger group is easy after the first few attempts. Use a complete trigger assembly as a guide if possible. First put the hammer spring on the hammer and wrap the arms around the flat part of the hammer, slide it into place between the rails and push it’s pin through the receiver to hold the hammer in place.
Next assemble the trigger and disconnect with the spring from your parts kit. On many US parts kits there is a round sleeve that hold these parts together and this system can be placed into the receiver easily. Or hold these parts together and drop them into the receiver and pin them as the hammer.
Drop the hammer spring arms down and hook the hammer on both sides. Test the trigger by holding down the trigger and cycling the bolt. Watch in the gap to see if the hammer is falling only one per trigger pull. Some US triggers are adjustable so the draw weight of the trigger can be changed to your preference.