One of the handy things about board games is that they are highly customisable. For instance, if you like the game Monopoly, you can adopt your own rules, use your own pieces, customise the board, and make your own cards. When it comes to making new cards, it's a good idea to replace all of the old cards with new ones, so that any new ones won't stand out among the old ones.
Design the new cards using your desktop publishing software. You can design them however you like, either to resemble authentic Monopoly cards, or to achieve a different look. The easiest method is to design them on sheets, then print out the sheets and cut the cards apart.
Print the cards on suitable card stock, in the range of 200 gsm (100 lbs). Make sure you use actual card stock, because regular paper is too flimsy for use as a Monopoly card. To be authentic, "Chance" and "Community Chest" cards should be printed on matt stock -- using burnt orange and cream yellow paper colour, respectively -- and title deed cards should be printed on glossy white stock. However, you can use whatever colours and surface texture you like. Note that title deed cards have print on both sides, and thus must be double-side printed or else run through a single-sided printer twice. Due to the heavy paper you will be using, you should consider going to a copy shop and using their heavy-duty printers.
Cut the cards from your printed stock using a paper trimmer, or have them cut for you by a professional. It is important to make clean cuts and size the cards correctly, especially for "Chance" and "Community Chest" cards, which need to be anonymous when they are turned face down. Copy centres often have paper trimmers available to use.
If you will be mixing new cards with old ones, measure the old ones with a ruler so that you know exactly what size to make the new ones.
If you plan to use your home printer, check your printer's user manual to see how heavy a stock it can handle, and what kinds of paper stock are safe for the printer to print on.