The sender(s) will solve your problems for you. The people in the future will already know how to do all this. They will know how to send and receive and will send that information in the very first transmission.

So how do they send a message to a past that doesn't already have the tech? They downgrade their signal and send it back to a regular modern day device (cell phone, laptop, whatever) of a specific person, a character you create who will be the spearhead for the whole operation, someone who has already started work on the time machine(?)

How is it technically done? Doesn't matter. We don't know how time travel works so it can be whatever you say (wormhole, faster than light speed signal, anything). Nobody asks how time travel works anymore. We just suspend our disbelief. How many Terminator movies have we seen without asking that question?