Firstly you need to strip the frame of all parts. That’s the easy bit.
Then you can either paint yourself, which involves keying the existing surface, spending lots of money on spray cans (such as Plasticote) and painting it. It can look good but there’s a knack to it that only comes with practice.
OR If you have a good local powdercoaters they can do it for you. If you live anywhere near Castleford I can recommend a good one. You have to make sure they are good with bikes as they will make a mess of it if they don’t mask it properly and protect threads etc. The finish of powder can vary from just as good as normal paint (as the place I take mine to can do) or horrible thick blobby finish which feels like the frame had been dipped in rubber.
OR send it to a place like Argos Cycles or Bob Jackson or Chromacoat and spend a LOT of money on a proper stove enamelled finish, which can look AMAZING but is very costly.