Okay so the first thing you would want to do is: Make sure it's around summertime cause that's the best time to work with cement. Never try this in the winter.
Last I checked 50 pound cement bags are about 10$, but this was a decade ago, prices may have changed slightly. You will still get a lot of bang for your buck if you go this route, so I recommend buying one if you have a place to store 50 pounds of cement.
If you are low on space like me, or don't want to carry a 50 pound bag of cement home from the hardware store, there is also the option of buying fast-dry cement (self explanatory), in a small 15 pound size bucket. Either way, store the cement, sealed, in a dry place, where it absolutely can't get wet.
Do not mix cement with water in the sink.
I feel I need to say this for some people, just in case. Your sink will get clogged if you do, as the cement hardens in the pipes. So will your bathtub if you decide to do it there.
The best place to mix cement with water is outdoors. The temperature should be hot but not too hot, as the cement will form cracks from hardening too quickly. Wear gloves and protective goggles.
I recommend using foam core (styrofoam with paper on both sides) as the material for your cast. It is easy to remove when the cement hardens inside. Do not use tape, except on outside layer of the foam core. Tape will leave imprints on the cement while it is hardening which will look very ugly, if you do.
Do measurements for the foam core, and draw lines where you will cut with a pencil or pen. Take your time. 1 inch thickness/scale is good for pure cement castings. Draw up a plan on paper or on computer, and use lots of numbers, height, length, width, before drawing on the foam and cutting/glueing the foam core. Your mold should be finished before you mix cement. I recommend a box cutter for cutting the styrofoam.
The best thing you can use for a cast is starch-based glue. (There are a couple ways to make it, google can help.) Glue seems weak but it will leave a nice smooth finish to the cement. Starch based glue is tougher than most other glues.
If you made a model previously, do not put your model inside of the cement.
Gradually add cement mixed with water (using a stick and bucket) to the inside of the foam while doing this until full. Pour it all in one session, but not all at once. You might want to lightly tap the sides of the foam with a hammer while pouring cement in, so the cement falls and it is tight and compact. You will also need a cover depending on what you are making, made of styrofoam, for the top of the cast, so the cement doesn't leak out. After filling with cement put the cover on top, and something heavy to keep it in place.
Cement should be evened from hardening. You can sand it down to make it even afterwards, however it will leave a coarseness to the material when touching it.
A good way to start if you have no ideas on what to make would be:
Take a large cardboard (usually called a plastic cup but it's really made of cardboard, idk why they named it that.)cup, and a smaller or same size cardboard cup. Start mixing cement with water in a bucket you can throw away, outside of your building, and then fill a small area, around 1 inch deep with cement mixture. put the smaller cup on top of the mixture and fill the sides with more cement until full to a little below the brim of whichever cup is lower height. This is just to make sure the cement doesn't expand upward and leak out of the outer cup, Tape the top of the two cups, tight, with strong tape together, since if the cement is hardening it will push the smaller cup upward, ruining the cast without the tape on it. Wait a few days for fast cement to dry, and a longer period of time for regular cement. You now have a vase.
Lastly be sure to clean up and wash your hands. Wash any cement off of you before it hardens. Use plenty of water to get it off.
P.S. IF temps are really hot, fast cement needs cooling while it hardens. Heck, even regular cement needs cooling. For a large concrete building, this would be done by spraying the outside with cold water occasionally. For a small cast I would recommend wrapping it in a bunch of wet newspapers. Leave a bunch of dry newspapers at the bottom, and point a fan at it. Don't do this if you have pets, or don't have a safe place in your house where you can leave the project. Leave it on the floor, not somewhere high up, because it could fall and make a mess. And definitely leave it somewhere you will not be walking around.
Here is a cast I made
@Troisnyx Please comment! xD (I know this is a very long newspost. I spent a lot of time making it shorter.)