Out with a bang
Explosives and explosions are difficult areas and there are ethical issues involved in providing too much information. Furthermore, it is probably inadvisable to go searching on the internet for ways to make things blow up. I can make a few observations here which may help writers avoid mistakes or misrepresentations.
Recent UK legislation has made it more difficult to obtain some of the materials which can be used to manufacture home-made explosives e.g. nitric acid, high-strength hydrogen peroxide and potassium chlorate. Licences are required to purchase and possess these chemicals under the Control of Poisons and Explosives Precursors Regulations 2015 - see the UK Government website for more details. Stories set in earlier decades could include home-made explosives using chemicals such as sodium chlorate and potassium nitrate which were then readily obtainable.
Health and safety
Attempting to make explosive chemicals such as TATP, mercury fulminate or nitroglycerin is fraught with danger and unskilled, untrained villains would be quite likely to poison or blow themselves up in the process. Telltale signs of such illicit activity, by the way, could be bleached or yellowed skin ( from hydrogen peroxide or nitric acid respectively). See also the section on Analysis for traces.
Nitroglycerin and mercury fulminate are unstable compounds (which is why they explode) and not suitable for carrying around in large quantities - only a slight shock is needed to set them off so don't have your protagonist taking them over bumpy roads or on the bus. These are not available to purchase so would have to be made - but see above.
Plastic explosive (e.g. C4, Semtex) is not something you can make at home and neither can you get it on eBay or from "serialkillersupplies.com". If your villain is using it they must have a military or organised criminal source from which to obtain it.
Lock, stock and shrapnel
Using an explosive to blow open a lock may be feasible but this is likely to send bits of lock and door flying in all directions - retreat to a safe distance, wear goggles and cover your ears.
Recently, Hollywood has realised that an explosion close by will leave survivors with permanent hearing damage, temporary deafness and/or tinnitus (depending on the magnitude and proximity of the blast) because of the very high peak noise levels produced. This is particularly the case with explosions indoors where the sound cannot escape and reverberates.
If you are planting a bomb, why would you have an elaborate visual timer counting down, and things that go beep, before it detonates? Hollywood regards this is as essential as a detonator and the main charge, in order to rack up the tension, - but it's one more thing to go wrong.
Flash or bang?
Note the difference between an incendiary bomb, which sets fire to things, and a blast, or HE (high explosive), bomb which shatters things apart. I wouldn't be writing this if the bomb which landed near my Dad in WW2 had been HE rather than incendiary.
Perhaps reassuringly it is not easy to make a bomb explode on cue. The device left on a tube train by a student in 2016 did not go off because the explosive had not been prepared properly and online bomb-making guides are no substitute for proper training (see https://www.theguardian.com/uk-news/2017/may/23).
The nuclear option
Several plots have addressed the idea of criminals/terrorists making a nuclear bomb (if they haven't stolen one from the former Soviet Union) and holding a city to ransom. This is not very likely as it would require the resources of a state, or state-sponsored group, to construct a reliable nuclear weapon and obtain the fissile material. A more credible threat would be a "dirty bomb" whereby radioactive material is wrapped around a conventional explosive. If detonated, this would contaminate a wide area and make parts of it unusable for decades or centuries, depending on the radioactive material used. Handing such material without stringent precautions would expose the perpetrator to potentially lethal levels of radiation but, in the age of the suicide bomber, this need not be a constraint.