Bombs are not easy to make... I mean.. the process is not difficult but making one that goes boom at the right time and not while you are making it or transporting it is a tad more difficult.
The bomb itself is not hard to make, but the detonation mechanism might need a bit more skill. But as I have said before, the IRA bomb that nearly got Margaret Thatcher at her party conference in 1984 utilised a detonator with parts from a VCR as the timer, so we're not talking rocket science. The bomb was planted several weeks before the conference.