You should complete an election form, SF 3109, Election of Coverage, and return it to your servicing personnel office. An election to transfer to FERS is effective at the beginning of the next pay period after your agency receives the completed form. An election to transfer to FERS is irrevocable once it has become effective. If you transfer to FERS, you then have a personal 30-day period to enroll in the Thrift Savings Plan or to change your enrollment.
Your spouse does not need to consent to a decision to change retirement plans. However, if 1) you have a former spouse who is entitled, by court order, to a portion of your CSRS annuity or CSRS survivor benefits, 2) the court order is on file at OPM, 3) the former spouse has not remarried before reaching age 55, and 4) the former spouse is still living, you cannot transfer to FERS without that former spouse's consent. Your former spouse needs to complete SF 3110, Former Spouse's Consent to FERS Election in order for you to be able to transfer.
OPM can waive this requirement only in very limited circumstances. If you don't know whether OPM has a qualifying court order on file, or want to request a waiver of the consent requirement, ask your servicing personnel office for Standard Form 3111, Request for Waiver, Extension, or Search.
Finally you may want to file a new designation of beneficiary form, SF 3102, Designation of Beneficiary, Federal Employees Retirement System, since CSRS designations are cancelled upon a transfer to FERS.
All of the forms mentioned above are located in the necessary forms section.