What is the proper way to make a spear with a fixed blade to prevent breaking as described by Vorac in his comment here?
This is the best way that I know of, courtesy of Field and Stream. The spear they made can be seen in the picture below. The guide I've linked is step-by-step.
You could just tie the knife on, but there is a high chance of it slipping, especially if the stick you're using is smooth (like Bamboo) and the knife doesn't have a lanyard hole to secure the binding in place.
Assuming you're using a flint knife, you could attach it this way. After tying it together, you could glue the binding using pine pitch or a similar adhesive.
Personally, I'd go with making a spear because that allows the knife to be used as a backup weapon and prevents it from getting damagedd. This guide by Wikihow covers nearly all the different survival spear types that I know of. If you intend to fish with your spears, you might find this guide on the Hawaiian Fishing Spear helpful.
If you're in a survival situation and need to get fish fast, this other fishing spear may come in handy. All credit for the guide and picture go to Hedgehog Leatherworks.
If you really want to use your knife as a spear-point, you could get a knife designed for such a purpose, such as the Cold Steel Bushman (picture below taken from company page). It doesn't break or chip easily, as can be seen from this independent destruction test.
Personally I wouldn't! You run the risk of losing/breaking your blade.
Better to use the blade to sharpen a stick and fire harden the tip (till it goes brown, not black) it or attach something you don't mind loosing like a flint napped blade.
Unless you plan to defend yourself from trees, I would focus more on the proper use of a spear fashioned from a knife.
The factors that must be considered are: (1) solidly attaching the knife and (2) how to use a spear without breaking the tip.
Attaching the Blade
One of the best fastening methods involves rawhide strips that have been soaked in water, wrapped tightly around the entire length of the handle and stick end, and left to dry. The characteristics of rawhide cause it to shrink as it dries, resulting in a wrap that is much tighter than could be done by hand.
Tie wire would also be an excellent choice, though not as readily available in the field. Tie wire is used to connect reinforcing bar (aka rebar) in concrete work. Use it like a twist tie, tightening it with a pliers as you go, and do as many pieces as will fit in a single layer. It can be found at hardware stores and lumber yards.
If rawhide or tie wire are not available, paracord would be another option but, due to the weave and characteristics of nylon, paracord stretches, making it difficult to maintain a tight connection. It is possible to shrink paracord slightly by keeping it wet, but using other materials would still be preferable.
Using the Spear
There is a direct trade off between blade sharpness and durability, so the sharper your spear, the more fragile it will be (assuming you're using a well honed edge). Blades are designed for slicing, spears for penetrating. The best thing to do, as mentioned previously, is not use a spear fashioned in this manner to fend off tree attacks. Get or make an archery target for practicing, and place something soft around the target.
If you would rather not be concerned about breaking the tip, fashion a spear head by hammering or grinding down a piece of rebar. The end result should look something like a large pencil (the yellow paint is optional).