Barbara Bush got to the Ellipse at exactly 11 yesterday morning. Her job was to put the top bauble on the National Christmas Tree.

The press got there at 10:30 and stood around on green plywood boards so they wouldn't soil the grass.

Frank LaGiusa got there at 10 to talk to the help. He's the one who designed the tree decoration this year, his seventh year, and there was a lot to explain. He is a lighting designer for General Electric, which has been running this show for 20 years.

"The theme is Hope this year," he said. "I started working on it right after the turn-on ceremony for last year's tree."

It will have green lights because Hope is colored green, and gold and white auxiliary lights will flood the tree in alternate 60-second cycles. This is made possible by a cross-fader that LaGiusa invented. Around the main tree, a 35-foot living blue spruce (the National Park Service insists it is only 30 feet, but maybe it grew during the year), are ranged 57 smaller trees, which also will sport white and gold twinkling lights.

This tree has been on duty since 1978. It is the third live one planted here and looks like it will make it. It is rather chubby. (Cut trees used to run 70 feet or higher, but somebody always complained about killing them.) The decorations, which will be put up over the next 10 days, actually are tied to wires so the tree itself doesn't have to do all the work.

The big problem of the morning was to arrange some portable steps so the vice president's wife could climb onto a truck bed and thence into a waist-high cherry-picker pulpit without having to change into slacks. Six people worked on that for an hour. Meanwhile, the bauble, a three-foot structure made of two interlocking diamond shapes, was taken to the top and tested.

LaGiusa himself was inveigled up there so a photographer could take his picture among the tippy-top branches. "I don't like this," he said. "I used to be in the construction business, and I slipped off a plank once in a church, 50 feet up. I even have trouble with stepladders."

One of his best designs was for a tree that had no lights because we turned them out for the two years of the hostage crisis. This one has plenty: a total of 11,575 watts, featuring a blanket of green light caused by 1,500 bulbs (every sixth one a twinkler) plus 500 clear bulbs, 500 gold bulbs and various others. In the past, trees have had dots, spirals, crisscrosses and splotches of light in several colors. This one will be draped in a diamond pattern.

Eleven o'clock. Here comes Mrs. Bush in a bright red dress. The photographers and TV crews stay politely on their green plywood. ("They assigned me to Brazil, but I said I wanted to do this instead," someone mutters.) Deftly, she climbs into the pulpit.

Where is Joseph H. Riley, chairman of the board of National Savings and Trust and president of the sponsoring nonprofit Christmas Pageant of Peace?

Here he is, trotting up at a bankerly pace, stuffing his watch into his vest pocket like the White Rabbit. He leaps nimbly into the pulpit with Mrs. Bush, and they take off, chatting as easily as if they were sitting in the back seat of a limo. With aplomb, she places the bauble. Everyone claps.

And for 20 minutes, they stay up there, exposed against the cold gray sky, talking calmly while one photographer after another is sent aloft in another cherry picker to get an aerial shot. It is surely the most accommodating performance ever seen at one of these prefabricated events. The Secret Service people look extremely nervous.

After all, when President Reagan lights the tree Dec. 16, he will do it by remote control from the White House back door, because the Secret Service doesn't want him roaming around outside at night, not even for a Pageant of Peace.