Paint for concrete/cement sign

Masonry paint. ISTR that Sandtex do sample pots.

That depends upon your skill with a paint brush. IME, there is usually some creep under masking materials and I get a neater job cutting in by hand.

Masking tape? It may depend upon how rough / smooth the concrete is.

I've only done something similar once but FWIW I used plioliote paint with a paint pad and dabbed v gently rather than brushing. Took several passes but the result was better than I reckon I could have achieved with a brush (albeit that's a low threshold).

Masking tape - always fresh stock as the glue can start getting hard and more prone to paint creep under it. Once you have masked off the letters use something like the wooden handle of a paint brush to press down the edge of the tape.

Consider using spray paint for the letters with a few light coats rather than trying to colour it in one go. The spray will cover any rough surface. Spayed graffiti on walls/concrete/stone lasts for years!

Remove masking tape as soon as the paint is dry