First, on the website of the Embassy of the country where you plan to go, check sample sponsorship letter. If the sample is (which is quite rare), then everything is obvious – make the letter pattern. If not, then proceed to the next step.
Sponsorship letter is written from a person who takes care of your expenses during the trip. It can be not only your relative, father, husband, brother, wife, but just boyfriend/girlfriend.
On an ordinary sheet of A4 paper in the upper right corner you need to write where is the sponsorship letter, e.g., "Embassy of the Federal Republic of Germany."
Below in the middle of the sheet is written the phrase: "a Statement of financial support".
Further text follows:
"I, (name, surname like in passport), birth date, birth place of residing (address of the permanent place of residence) hereby undertake to provide my daughter/wife/girlfriend/... name, the name of the person to whom you are writing the sponsorship letter, as in the passport, date of birth, place of birth, address of permanent residence, full material support for the duration of his/her stay in the territory...(e.g. Germany).......(put date of travel).
In addition, pay or provide reimbursement of expenses related to his/her treatment and, if necessary, with the return of the native.
After the text of the statement do not forget to sign and date.