This tutorial assumes that you already have your entrance tiles prepped; all of the tiles surrounding and including the entrance tile have been dug down to rock.
Tunneling in creates a new entrance with a 20 ramp slope down.
Improved compass and tool tip mods will help you with corner height.
For the best results, I use 30 or 33 slope between top and bottom corners of the entrance tile (outside) depending.
30 is the default height (slope) from floor to ceiling if mining forward.
If you're considering having buildings in your mine, 33 is the default height height (slope) of a first floor of a building from bottom to top. Subsequently it is 30 for floors 2 and up. This is wall height and doesn't take into account the height of a roof. Roof height if the width of the roof is 1 tile is 20 slope high. Roof height if the width of the roof is more than 1 tile (vaulted ceiling) is 38 slope high.
How to Make a Flat Entrance
Method 1:
Align the floor and ceiling corner heights (inside) with those of the entrance tile (outside). Mine out from the inside. You should now have a flat entrance.
Method 2:
Tunnel in, which puts you down 20 slope, tunnel up the next tile inside (reversing the 20 slope), then mine one tile in to get a flat tile. Fill in the down and up slope tiles, then mine straight out. You should now have a flat entrance.