First of all,

I gonna use a <30mW Laser, to not burn your eyes use Laser-safety-goggles and become sure no one get harmed by your actions. While working on the tweezer it's safer not to wear an metallic or reflective stuff and use anodized tools.

A few more details on this development, I have built up that construction in recent weeks at the University of Tübingen for the beginner training in physics. This structure will cost quite a lot of money but who can afford it will surely have a lot of fun with it.
In this structure, polystyrene beads are used as colloids. Since the diameter is much larger than the wavelength of the laser I will add only the geometric explanation of optical tweezers.

The distances between the individual optical elements are only of secondary importance. But I will explain every part of what is important in the beam path as the laser beam and look lost.

I know this instruction is not very good but with your help and some time it will be helpfull.

Step 1: Adjust the Laser

You should see a center laser dot on all your lenses and especially on the objective lens.
And a center dot at your ccd-image. To achieve this, use two of the three mirrors. The second mirror is set piece by piece so that you can see a central point on the CCD. Then, the first mirror is adjusted so that the aserstrahl objetive re-center hits. I have achieved the best results when I've adjusted the levels simultaneous with two hands