How can I apply for disability benefits from past years transferred from my mom. She is already approved.

Going back several years is a complicated process..  You would need to do a T1 adjustment for your mother for each year applying the disability tax credit to her returns first.  Then do a T1 adjustment for each year for yourself showing the transfer of any of the credit she did not use.  Be aware that if she had other deductions, the disability credit will be applied first.  If she uses it to reduce her taxes, the other tax deductions may be reversed.

It may be easier to send CRA two letters, one asking CRA to reassess the specific years affected by the certificate and to transfer the unused credit.  Your mother will need to send that one.  You would then need to send a letter from you with it, asking to have the remaining credit applied to your tax returns, and have them reassess yours as well.

http://www.cra-arc.gc.ca/E/pbg/tf/t1-adj/t1-adj-fill-15e.pdf

The criteria for the transfer is below:

You may be able to claim all or part of your dependant's (other than your spouse or common-law partner) disability amount (line 316) if he or she was resident in Canada at any time in 2014 and was dependent on you for all or some of the basic necessities of life (food, shelter, or clothing).

In addition, one of the following situations has to apply:

• You claimed an amount on line 305 for that dependant, or you could have if you did not have a spouse or common-law partner and if the dependant did not have any income.
• The dependant was your or your spouse's or common-law partner's parent, grandparent, child, grandchild, brother, sister, aunt, uncle, niece, or nephew, and you claimed an amount on line 306 or line 315 for that dependant, or you could have if he or she had no income and had been 18 years of age or older in 2014

http://www.cra-arc.gc.ca/tx/ndvdls/tpcs/ncm-tx/rtrn/cmpltng/ddctns/lns300-350/318-eng.html