Because tutorials on making jungle scenery are so easily available off the internet (links listed at the bottom), I will eschew the details and say simply that investing in a hot glue gun can be one of the best decisions you can make in the hobby.
When I first started doing them I wanted to imitate the readymade pieces sold by Miniature World Maker, which I had seen arranged to great effect on the HKSW site. So I ended up making large rectangular bases bearing four to six trees. The trees are plastic and can be bought in exorbitantly-priced packets at B2S, or in super cheap bags at Jatujak. My many critics have pointed out that I should really drybrush the trees, and give them a coat of matt varnish to lessen the shine, but quite frankly I’m too lazy to.
On the finished base pictured below is a lone battling bastard desperately fending off a typical bayonet charge. The Tommy gunner is from Sgt. Major’s Yanks range, while the surrounding Japanese are Eureka.
The idea quickly proved unfeasible, however, and I resorted to doing what many other people were doing; i.e. making smaller, irregularly shaped pieces that can be arranged any which way you want on the table.
Finally, here’s a couple of useful links for people looking for a step-by-step guide.