Hand engraved stamps:
First the piece of tool steel must be cut, machined etc. to the desired shape and measurement. The upper part of the sides get a conical shape. The remaining frontal area must be a little wider than the actual design. This is because there must be some metal left for the side flanks of your design. Dont forget that you must work with the mirrored logo or text! The front face of your steel piece must be absolutely flat and finely finished. Draw your design on it and cut with a V-section graver. You easily can prepare an onglette this way. Just give the graver three flat grinds which form a tip. Clear that such a graver cannot be used for normal engraving. This graver does not have a heel and can only be used to push down and scrape the side walls. In order to make it easier you could first cut away the steel in the center of the widest areas with a round graver. It is very important never to lower the handle as your graver would ruin your work with the sharp lower edge!! Try to obtain smooth sides on your stamp right from the start. Cut away small chips only and use a scraping motion with one side of your graver. Use a 10 power hand held lens for this as you want to look from the sides and see your tool work. The hardest thing is to cut nice corners, especially where more than two lines meet... The end result is a stamp that has smooth cut flanks and a very narrow and even edge at the top. You can check the design with an ink-pad and a piece of cardboard. If finished harden your stamp.
See samples here (The ones with a flat ground are CNC-engraved. Hand cut stamps never have a flat background!)
CNC engraved stamp:
You need a drawing or design that can be scanned and then vectorized. For this several software programs are available. You can use CorelDraw or Inkscape for example. Note that your vector design must have closed outlines. Otherwise the CAM software cannot calculate correct milling paths. Then it can be scaled to the correct size and flipped. The steel part must be prepared roughly. The piece can be shaped in a way that it can be used in a press or by hand only. The front side can be milled in the CNC to be exact flat. It should be then ground smooth and the point of origin can be set. Milling paths must be calculated in the CAM software. First the stamp will be roughed with a single flute cutter that has a small tip-off. This ensures that the tip does not break all too easy. The metal is roughed in multiple layers at a very low feed rate, but high RPM. The final cutter has almost no tip-off and therefore can cut relatively sharp corners. After the milling process you can file away excess metal and harden your stamp.
Maybe it is best to just draw the design and get it produced professionally.