Ok I posted a tut already but it might sound too complicated so here's a step by step less complicated guide

Step 1: note down your coordinates and write them in the form (x1, z1)
Step 2: throw eye of ender
Step 3. Note down coordinates of where the eye of ender landed and write them in the form (x2,z2)
Step 4: write down your coordinates like this: (z2-z1) = m(x2-x1), where m is the gradient which you can now find
Step 5: solve for m (the gradient)
Step 6: write another equation like this: (z-z1) = m(x-x1) but this time write you value of m in, leave x and z as unknowns and sub in one of your coordinates I.e (x1,z1)
Step 7: write equation in form z=mx+c (once you have an equation this is easy, mx is just a number being multiplied by x and c is just a number.

You have now got one equation!

Step 8: run quite far away from where you were (the further the better) and note down your new coordinates
Step 9: repeat the above steps until you have another equation
Step 10: once you have your two equations in the form z=mx+c do as follows

Make mx+c =mx+c (at this point you will know the value of m and c)
Solve for x
Use this value of x in either of the equations that are in the form z=mx+c
And now solve for z

You now a value of x and a value of z, these are your coordinates for your stronghold, all you have to do is get there and dig down. Enjoy!

For the curios ones..
Why this works:

You can find the equation of a line if you have two coordinates. (In this case where you are when you throw your eye of ender and where your eye of ender lands)

The eyes of ender will travel in one direction until you have either passed the stronghold or are directly above it.

If two eyes of ender are thrown and are traveling along the same straight line they will only ever cross each other once. (Another way to think about this is to draw to straight lines with different gradients, they will cross each other only once) at this point the coordinates of the lines are equal, as they are in the same place. Therefore if we have two equations we can solve for the x and z values by making them equal to each other (because they will be equal when they are in the same place)