Agarose ou Acrylamide? Other?
Ethidium bromide included or not? This is a bit vague, but to start with the most common, agarose:
1- Melting the agarose: if in microwave be careful not to overboil (take a large flask, do not overheat, beware when handling liquid from the microwave). If you use a hot plate, the main thing to avoid is to burn the agarose.
2- cool agarose down, nothing special, but do not cool too much (then go back to 1)
3- If you add EtBr, it’s now time to. beware to wear gloves and goggles, work in the designated area (or in area easy to clean in case of spill). Do NOT breathe vapors. Mix well.
4- Pour agarose into gel cast, and add comb. If EtBr is is in the gel, be carefull of spills. Before pouring make sure that your cast is leakproof (depends on the model, ask someone who has some experience with that particular model) and on a flat surface (to keep gel thickness constant. Then let cool down enough (I usually count ~10min after the gel has reached the milky color it has when it is set.
Let the agarose cool till you can touch the flask with your bare hand, pain-free, before adding the EtBr. Swirl the flask every now and then as it’s cooling too, to avoid getting polymerizing regions against the glass.
I was doing Southern blots; I don’t know how important it is for normal check gels and whatnot, but when casting, make sure your tray is level. Pouring agarose when it’s too hot will cause the plastic tray to warp, giving you areas of uneven thickness in the final product; this may affect migration rate and transfer.