You take a bowling ball size lump of highly enriched Uranium and Plutonium. This has to be machined to be almost perfectly spherical and of constant density - you'll see why later. It has a hollow clyinder that is used to hold tritium rods that are fired into the ball just before just before implosion and neutron bombardment.
You send the tritium rods into the ball then implode this combined entity with a shaped charge (say surround the ball with an inch and a half of a two layer (slow than fast burn) C4 derivatives (miltary grade explosive with an extremely high burn rate - say 7,000 metres per second on the outer and 10,000 metres per second on the inner - with tungsten-rhenium drivers and on the otherside of this charge facing the ball is a 1cm thickenss of Berllyium - touch a 1mm thickness of U-235 surround a 10 kg mass og plutonium 239) that is ignited by 240 special denotators linked to krypton switches on precisely measured lengths (typically 1 metre) of cables to your denotator box containing high voltage capacitors linked to a timer. Your charges have to go off within a nanosecond (called a shake) of each others to crush the ball into an almost perfectly round much smaller ball. You want the bowling ball crushed to the size of a tennis ball. If the shaped charges aren't perfect your bang bang is goofed.
So the key challenge is mechanical - you need to refine P-239 (and avoid unstable P-240 which is likely to go off prematurely) crush it with a million atomspheres of pressure with a precisely shaped spherical wave that must be re-shaped to a planar wave implosion
About 5 shakes after your implosion you shoot high energy neutrons (from a zipper - a small cyclotron through a lithium-deuteride disk the size of your palm about 8 mm thick into the crushed ball though the Beryllium. This means you slow neutron source (travelling about 10% light speed hit the compressing P-239 core (already 10 times denser than lead and imploding) in trillions of places at once and fall under the Strong Nuclear force of the P-239 and get captured - turning it into unstable P-240) - bingo Nuclear reaction. The P-240 decays into two simpler atoms - but annihilating either a proton of a neutron and releasing 3 neutrons in the process - that will further incite P-239 or P-240 - runaway chain reaction follows at a geometric rate.
Crushing the ball into a smaller size raises its density above critical mass (which really should be critical density). The tritium explosion is racing in and out at 7,000 feet a second - but reaching chaing reaction takes less than 200 shakes - so the pressure wave of the shaped charge travels less than 2 feet before fission (run away chain reaction) has intiiated.
About 10% of your mass will be annihilated so a 10 kg ball releases about 1 * (3 * 10 ^ 8) ^ 2 newtons of energy.
Now if that ball if surrounded by a compressed source of hydrogen the fission reaction releases enough energy as heat (bewteen 10 - 100 million degrees celcius) to trigger a fusion reaction so you've gone from an atomic weapon (fission bomb) to a hydrogen bomb (fusion weapon) that releases between 10 - 1000 times more energy.
Lead doesn't fuse - it takes more energy than it releases - you need a really massive, and atomically unstable atom what wants to radioactive decay - to trigger fission. All you are doing is suping up the decay rate a trillion times or so!
(Source: Surprisingly accurate - Tom Clancy - The sum of all fears - pg 793 - 798)
