Measure the desired height for the table and draw a straight line on its legs. If you are a first-timer at this, mark another line an extra centimeter longer as well. You should now see two marks on each table leg.
Turn the table on its side and mount the first leg on a sawhorse or work bench. Clamp or nail a piece of scrap wood onto the first leg, along the guide line you just drew. This temporary board will guide the saw directly instead of your hand and fingers being too near the guide line. (You still need to hold the leg down, but it won’t be close to the handsaw anymore.)
Use the practice mark to carefully cut a notch into the wood, making sure that the line always remains visible to you.
Angle the blade so that it is 45 degrees to the leg. Start cutting but do not use full force as an Ikea side table is usually made of MDF material that snaps easily. Nearing the end, slow down to smaller strokes to avoid splitting the leg.
Practice on all four legs if necessary. Alternatively, you can saw off all legs to the actual desired table height without practice.
Remove all wood scrap saw guides. Sand all sawed ends. Let the table stand to test for uniformity in height and stability and sand further if needed to adjust stability.
Glue furniture coasters to the sanded leg ends to give the table a finished look and protect your floor.