BuyorSell (I think) in the past posted a photo of a clem growing up a spiral of copper plumbing pipe around the trunk of a tree so that the clem followed the loose spiral around the trunk. You could also hang strings or netting from the tree if the lower branches are large enough to support the weight of the clematis.

It will also depend somewhat on the kind of tree. Since clematis aren't fond of root competition, certain trees with shallow thirsty roots will stunt the growth of the clematis unless the clem is planted in a pot (and maple roots will grow up into the pot through the drain holes of a pot sitting on the soil surface!)

My copper spiral doesn't actually work very well. You need something like metal grid fencing wrapped around the trunk or part of it.

or something like this trellis

Here is a link that might be useful: trellis at Amazon

Do you remove the plant from the trellis in the pot and then feed the vine up your own trellis...to start its climb? I realize that this might be a silly question but this is all new to me as a first time gardner.

Rather than buy expensive tree trellis, I use leftover scraps of my horse wire fencing - any kind will do - to wrap around a tree for the clematis to climb on. Once they cover the wire, you never even know its there. You can buy rolls of all kinds of hardware cloth, etc. at Lowes. I would think plastic netting would work also.

The plastic netting doesn't work well as it breaks apart when you pull the vines down after pruning.

Wire livestock fence certainly works great.

Yes, you remove whatever stake or trellis is in the nursery pot.

Metal mesh works great for me. I grow Pink Fantasy and Venosa Violacea up the tree trunk. Make sure your clematis is planted a couple feet away from the tree and train it toward it, or it will never get enough water.
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That's a beautiful picture "grow darnit" -- thanks for all the help on this. And how do you prune Nelly Moser?

Grow Darnit, My Venosa Violacea grows more than 12 feet tall and at least 8 feet wide after being cut to the ground each year. Keep an eye on yours so it doesn't swamp Pink Fantasy which is a much less vigorous grower.