I need to drill 2 holes in a concrete wall, about 1.5m apart and I need the line between them to be as horizontal as possible.
I have a level, not that big, and few other basic tools.
How can I align them properly?
Make a plumb bob by hanging a heavy and centered weight from a chalk line and snap it in the center of your desired holes.. Now pick a point on the wall that will be your left hole and two points on your chalked line. Use a wire and measure the distance from the hole to the upper point on your chalk line. Draw an arc on the opposite side of the wall. Repeat for the lower point on your chalk line; where they intersect is the location of your other hole.
If you have a shorter level than the distance between two hole positions you can find yourself a piece of wood that is say 2 meters long that can be seen to be nice and straight.
Hold this length of wood up against the wall in a horizontal position and set your level atop it. The wood can then be repositioned till the level shows a "level" position on its bubble. The wood piece effectively extends your level and should work very well unless the level is one of those really cheap short 5cm to 10cm jobs.
Once the wood piece is positioned properly you can draw a line along the top and have a good level reference. You can pre-mark two positions on the wood piece that are 1.5 meters apart so that you can know just where to draw short marks instead of having to mark a huge line across your wall.
One method to do this if you do not have access to a long level is to use a water level.
Get yourself a length of clear plastic tubing from the hardware or auto parts store. For your application it would be good if this tubing was 5 or 6 meters long. Now hold the two ends of the tubing up in the air with the remainder of the tubing hanging down. (It can be extremely beneficial to have a helper for this process).
Fill up a good portion of the tube with water but keep maybe a quarter or third of a meter of unfilled tubing at each end. The ends of the tubing need to stay open to the atmosphere. The water top in each upraised end of the tube will be level with respect to each other. As long as the two tube tops are held at the approximately same height you can move this around to use as a "level". If it is a little hard to see the water top through the tube because the tubing is not really clear you can mix food coloring into the water to make it easier to see.
Mark the position of where you want one of your holes. Then measure over your 1.5 meters distance from the first mark at an approximate horizontal position and make a vertical tick mark. Now with your helper position one end of the tubing with its water level top matching the first mark and the other one near where the second vertical tick was placed. Once equilibrium is reached and the water level is at the first mark you can make a horizontal tick at the water top across the vertical tick. This will mark two spots that are level with respect to each other.
If it is really hard to estimate the "approximately level" place to mark the vertical tick you can use a string from the first mark and measure over the 1.5 meters and draw an arc instead of the short vertical tick. Then when the water level is brought into position you mark across the arc instead to get the level position for the second hole mark.
Kind of a small version of this principle...
It works because turing the level around each time (in theory) cancels out any inaccuracy in the spirit level.
Of course it requires very careful observation of the bubble, every time, plus careful marking.