What is your specific difficulty? The terminals ("pins") are typically inserted into the housing via hand. If they simply won't go into place, then you may have distorted the terminal with the crimper. If the crimping tool die is too wide, the resultant crimp won't fit into the housing cavity. Use a narrower crimp if this is the case. If you crimp too hard (excessively low crimp height), then the terminal can take on a banana-like shape. The solution here is to either adjust the crimper (if possible) or simply not squeeze the crimper as hard.

If the terminals insert into the housing but won't stay in place, check the little barb on the terminal - it's really easy to flatten them while assembling the connector.