“Starting” is a bit ambiguous.
But I suggest that you first simple read the ISA specification. It’s surprisingly readable and if contains justifications for some design decisions.
After that, buy a small FPGA board and run a picorv32 CPU in it. Or one of the many other RISC-V soft cores.
It’s a small (tiny!) FPGA board that’s still large enough to run a RISC-V CPU. It’s a full open source flow. And there are plenty of examples.
Get that first LED blinking!
Oh, and https://www.nand2tetris.org is great. You implement a simple CPU, and though the language isn't a real-world one you learn enough to probably be able to implement a similar CPU on an FPGA using VHDL or Verilog.