Years ago Doc made a spike for me from a screw driver...he ground down the edges to form a point on a grinder. If I just use that on the underside of the containers I find the plastic splits too easily...
I have found that by heating the tip of the spike it actually melts the plastic creating a clean hole and no split.
The heat source I use is simply a candle in a tuna tin. After a few minutes I find the spike end is hot enough to put about 8-10 holes in each 'pot' without re-heating.
This procedure needs to be done outside on a breezy day as the burnt plastic no doubt is giving off nasty fumes that I'd rather not be breathing in.
Also keep all this equipment, the spike, the candles
and the matches away from young children.
Because the holes are in the base of the 'pot' it's best to keep them in a mesh tray to allow for better drainage...not flat on the surface. You could add a few holes at the bottom edge on the sides if you have to keep them on a flat surface.
If you don't eat foods that come in these plastic containers ask your friends for theirs...at garden club the folks know I'm always planting up something and give me their containers by the bag full.