Cover all the holes with your fingers as shown on the diagram and blow. A note somewhere around middle C should sound out. Tune it by moving the mouthpiece in or out. Use a piano or C harmonica  as a reference. A real tuner is even better.

After the C is in tune, lift your right pinky to uncover the first hole toward the end. Hopefully the blown note with that open hole is D. If it's flat, enlarge the hole with a scraper or a reamer until the note blows a correct D. A bigger hole makes the hole go sharp.

If the note is sharp, use some of that thick superglue(crazyglue, cyanoacrylate...) in the hole. You'll want to make the hole smaller. A smaller hole makes the note go flat.

When you're done with the D hole, move successively up, tuning each hole one at a time. Remember to be patient. Put it aside and leave it for an hour or a day if you get tired or feel your patience/attention span wearing thin.

Hole sizes shown are only approximations. Your instrument may be quite different.

Once all the holes have been tuned to their proper pitch, the clarinet is done. Try it out. Blowing while uncovering the holes in a linear fashion from the end on up will give you a simple diatonic C scale, plus three notes. Notice the thumb holes are both covered by the left thumb and fingered in reverse. I personally feel it's more comfortable this way. Keep practicing.

You should end up with something as crude looking as this.--->