How to hide the bride for outdoor ceremony?

Ask your venue what they usually do. They should be able to help.

Is this a rental venue, or you/a loved one's personal land?

If it's the former, ask their assistance.

If it's the latter, you could consider something similar to what dancers use backstage at competitions - essentially a portable changing room. Or simply a pipe/drape situation, where you get two tripods/stands and drape/hang fabric between them.

This is a basic idea of what I mean. You could get more opaque drapes.

So we are having our wedding in the backyard. The ceremony will be all the way in the back of the yard. Since it will be such a long walk, we decided to set up a small tent with sides for us to stand in that will be closer to the ceremony. Maybe a tent off the side will be a good idea.

I've seen stuff like this available for rent frequently.

Have you considered waiting a few blocks away and then having someone drive you there once it is time? I've heard of people renting a limo or bus for an hour ish so they can ride over when it is time, and then exit the vehicle and walk down the aisle. A tent is also a great option!

They have doors you can rent for this type of occasion that set up and open when the bride is ready to been seen

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This is what I would recommend. A pop up canopy and you could hang some drapes to prevent guests from seeing you.

I would also recommend renting doors or something like that to give you the actual entrance feel.

If those aren't an option, hanging out in a car/limo/bus/whatever beforehand would be a good plan.