I really like the thick-looking pads in this image: www.dallasbonsai.com/v/vspfiles/images/tomMar05_2.jpg

What is the best way to thicken the canopy of my Chinese Elm?

Also, to stop the tree from looking too disproportionate, I think it'd be a good idea to thicken the trunk. How would I go about doing that? I read that having the tree in open soil would help this, but I'm not too confident with doing that, and having a couple of young dogs around would worry me slightly.

I'm only a beginner, so please go easy on the advanced terminology

Thanks

www.bonsaiempire.com/tree-species/chinese-elm
www.bonsaiempire.com/basics/styling/advanced/trunk
www.bonsaiempire.com/basics/styling/pruning

So regular pruning will cause the branches to back bud, thickening the canopy?

I keep seeing different advice for maintenance pruning Chinese Elms, some saying every 2-3 months and others saying 1-2 times a year. Would you be able to clear this up?

And, should the top + sides be pruned more often / vigorously to ensure even growth?

Valkyrie2271 wrote: So regular pruning will cause the branches to back bud, thickening the canopy?

Proper pruning will create ramification, which will create more branches and more leaves, making the canopy fuller.

I keep seeing different advice for maintenance pruning Chinese Elms, some saying every 2-3 months and others saying 1-2 times a year. Would you be able to clear this up?

Hmm.. not too experienced with Chinese elms. I prune the first flush (when it has hardened off), then I prune again when new shoots have fully grown, if / when needed. I do not prune where I need more girth / longer branches etc.

And, should the top + sides be pruned more often / vigorously to ensure even growth?

No, you prune where and when it is necessary. Depending on the shape of the tree, you will let a branch / branches grow freely to get more tapering,
You may want to let branches grow freely to develop them, then cut them back, let them grow again, also to get tapering.
A bit much to explain in a forum post; keep studying, there are many sites with in depth information.