When Pluto was discovered by Clyde Tombaugh, how did he know,where in the sky,to look?

He didn't.

While William Pickering of the Harvard Observatory and Percival Lowell and others had tried, in the 1890s through to the 1910s, to predict positions of a hypothetical 'Planet X' that was affecting the orbit of Neptune, no such targeted searches had been successful (though it would later be demonstrated that, quite by accident, 134340 Pluto was in fact present on some of the images).

By the 1920s the Lowell Observatory had decided to try a systematic search using an astrographic telescope and a blink comparator. The idea was to take images of a given patch of sky on different nights, separated by a few days, and then compare the plates to see if any object changed position. Somewhere I'm sure I've seen quite a detailed description by Clyde Tombaugh of the design of the survey but I haven't been able to turn it up on some web searches (I think the one I remember was a book I borrowed once from my local public library). There was an article published in the January 1932 issue of The Scientific Monthly by Roger Putnam and Vesto Slipher describing the Lowell Planet Survey and the search for Planet X but it would appear that an electronic version is not available (except perhaps if one is a member of American Association for the Advancement of Science).

Anyway, as I recall, the survey focussed on the ecliptic plane, that is, the band of sky that traces out the Sun's apparent path during the year (in fact the plane of the Earth's orbit around the Sun) and near which the other planets are always seen. So the objective was to cover the entire ecliptic, out to something like 5 or 10 degrees either side. The astrograph took images on 14-inch x 17-inch glass plates. The regions of the ecliptic targeted on any given night were mainly calculated to give the biggest motion of any objects at distances around 30-50 AU. Again I can't remember the details properly but I think this meant an opposition search as that is when the Earth's motion of 30 km/s (about 2.5 million km a day) would give the most parallax shift of a distant object. So each night the aim would be to take images near the opposition point, that is near the ecliptic and directly opposite the Sun in the sky.

Taking the images was boring enough but the really yucky part of the job was the tedious work of examining the plates with the blink comparator. You loaded two plates of the same region of sky, taken a few nights apart, and viewed them through a projector with an eyepiece and a mirror that allowed you to switch back and forth between the two. The plates would first have to be aligned on the background stars so that they stayed fixed when you switched and then you would have to proceed methodically across the plate in a kind of raster fashion, clicking back and forth, to try to catch sight of anything that shifted position.

It wasn't something that Slipher and the professional astronomers wanted to devote their time to so they hired someone, Clyde Tombaugh, to do it.

In the end, it took less than a year for Tombaugh to find 134340 Pluto but he continued the survey for many years afterwards, covering the entire ecliptic. He said that he found it progressively harder to do the blink comparator work as the years went by. He discovered lots of other asteroids and comets during the work, but no other trans-Neptunian objects. 134340 Pluto is considerably brighter at the present epoch (including 1930) than other objects out there and thus they didn't show up on Tombaugh's plates.

Modern asteroid surveys work on the same basic principle, though these days the blink comparator part is done with a computer. You can do it yourself with the low-cost software Astrometrica and image archives like SkyMorph.

by observing gravitational perturbations on neptune as it orbited past it.

once the general location was known, pluto was isolated by the blink comparator method.

(comparing photos of the same piece of sky taken at different times -- stars in the photos remain stationary, while a nearby body such as a planet can be seen to move from one photo to the next.)

He didn't he just looked a long the ecliptic for years until he found it

Mickey Mouse told him where to look.