I am a hobby carpenter of about 7 years experience but I only started carving about 6 month ago. I fancy that anyone who has chopped out mortices and tenons can cut basic shapes. i use flat chisels obviously for straight letters (I, L, M, N etc). And I started with a cheap set of gouges topped up by some ancient second hand ones. For the letters I copy a font from Word or elsewhere and print it at a suitable size. I cut the letters roughly around as using the word as printed seems not to give entirely suitable letter spacing. I then glue the letters to the wood. Any English hardwood seems ok, definitely oak for outdoors. Don’t practice on pine, it is awful, nor foreign stuff which seems splintery. Use simple pva glue diluted 50/50 with water, otherwise it is hard to get the paper off at the end, just dampen to remove it works a treat.
To start I used a thin router bit (about 3mm) set about 5 or 6mm deep and went around the outline of the letters leaving about 2 or 3mm to chop away next to the letters. Now with an 8mm or so gouge out down to the bottom of the router groove being careful about not hitting the safety strip around the letters. I don’t worry about the ‘rough hewn’ look of the gouge marks - see below. However one could just as easily router out all of the background. Chopping it all out would be pretty tedious. I did the oak leaves with a vee gouge.
To finish the letters chop down at about 60 degrees along the letter outline to the depth of the router groove then cut under to clear the waste away. The only tricky bits are inside of curves (if you avoid using serifs) as the outsides of curves can be cut with a small flat chisel.
This is an example of what I mean
. Although he does not use a router. Nor do I now but it helps to establish depth. One last thing - use seriously sharp tools. See Mary May on YouTube for sharpening gouges. Any further questions then you can PM me (I think). Good luck - go for it.