Writing is prohibited on Yom Tov. However, it is permitted to write for a kidushin, divorce, receipts, testaments, gifts, pruzbal, shitar of evaluation, shitar about food for daughters, shitar of chalizah, shitar of miyun, shitar of picking judges, edicts of Bais Din or governmental documents.

Moed Katan 3:4

You should not write loan contracts, however, if the person will not lend to you otherwise, you could write one.

You cannot write sefer torahs, tefillin or mezuzos on Chol Hamoed.

You cannot fix letters even for the sefer torah that is in the Azara. R’ Yehuda says you can write tefillin and mezuzos for yourself and spin techeilos for your own tzizis.