good on you for planning ahead.
Cervical Caps and Diaphragms have a typical failure rate of about 20% (of 100 couples using the method for a year, 20 pregnancies might occur), but they are a good method to use with condoms. Both cervical caps and diaphragms need to be fitted at your gynos, because they come in different sizes, and because your health care worker will check you inserted correctly. Both need to be used with spermicidal jelly, which can cause irritation in many women.
To lower your pregnancy risk even further when using condoms and the cap / a diaphragm, it might also be a good idea to watch your fertility through taking your basal body temperature every day and observing the changes inyour cervical mucous. - You might then completely abstain from all sexual activity on your most fertile days.
For more on Cervical Caps and Diaphragms, check out Planned Parenthood: Cervical Caps & Diaphragms, which has insertion info, too.
"it may look like i'm moving but i'm standing still."