To set up a speed dial option, make sure that the phone system is in standby (no handsets in use). Make sure that the number you want to have as a Speed Dial number is in the Directory. You can type in the number (up to 30 digits) and then press Select (shared key with Menu to the upper right of the arrow keys). Alternatively press Menu and use the up/down arrows to navigate to Directory. Press Menu/Select twice. Type in the phone number. Press Select to move to adding a name with the number key. Press Select to Save. You can also add a number from the Redial list. See the abbreviated manual on page 7. Press Off/Cancel to return to the previous menu list. Press and hold Off to exit the menu system completely.
Now, press Menu (if you aren't in the menu) and use the up/down arrows to navigate to Directory. Press Select. Scroll to Speed Dial and press Select. Select a speed dial location and press Select. Scroll through the directory list and press Select when the correct entry is highlighted. Press and hold the Speed Dial number (0 or 2-9) to dial this number. See page 8 of the manual.
The abbreviated manual for the CS6629, CS6629-2, etc is available here: https://phones.vtechcanada.com/media/document2/products/%7B76B9FBDF-8471-4E06-BE52-B0E8A258E5F8%7D/CS6629-3_CAAUM_3_912.pdf
I hope this helps.