Everyone knows about this, but I wanted to re-use this for my next series of custom web part posts. 🙂
I have already developed and deployed  the web part (WebpartStepBiStep.wsp) using STSADM command, you can see my previous post here. Now we will see the steps:

Step 1: On the SharePoint site, click Site Actions –> Site Settings –>Modify all site settings

Step 2: Site Settings page, click Web Parts under Galleries

Step 3: On the Web Part Gallery page, click New

Step 4:On the New Web Parts page, select the check box next to the Web Part that you want to add, and then click Populate Gallery

Step 5: Web part will be showing on the web part gallery.

Step 6: Click Site Actions, and then click Edit Page

Step 7: Click Add a Web Part

Step 8: In the Add Web Parts dialog box,  select the check box next to the Web Part that you want to add, and then click Add button

Step 9: The web part will be shown on your page.

Step 10: If you want modify the appearance and layout then Click Publish.

Step 11: That’s all, we are done.

Reference:
http://support.microsoft.com/kb/939306