Purchase a lower receiver, The picture above is an Anderson Stripped Lower Receiver and a Palmetto State Armory Complete Upper Receiver.
I picked my stripped lower at Cabelas up for $50 but it can be ordered online and shipped to a dealer near you.
The complete upper can be found here: https://palmettostatearmory.com/psa-10-5-carbine-length-5-56-nato-1-7-phosphate-12-slant-m-lok-upper-with-bcg-ch-5165448745.html
The lower parts kit for this build can be found here: https://palmettostatearmory.com/psa-sba4-moe-pistol-lower-build-kit-black.html
Its running the Holosun 510C optic. https://www.amazon.com/Holosun-HS510C-Reflex-Circle-Holographic/dp/B01MT5RJLD
Here is a link to help you put the lower receiver together:https://www.youtube.com/watch?v=z7MGhlwkLk4&t=150s
Here is the link for the sling: https://magpul.com/catalog/product/view/id/5272/s/ms4-dual-qd-sling-gen-2/category/52/?mp_global_color=118
Take to the range and zero the optic. Hit me if you have any questions.