Pronounce the numbers one through 10 once on your own. Then have the students follow you, one number at a time. "Uno", or, one, sounds like "oo-noh." Two (dos) sounds like "dose." Three (tres) sounds like it is written but students may have trouble rolling the "r." Practice this by having them hold the tip of their tongues to the palate while vocalizing and forcefully blowing out air. Four (cuatro) sounds like "kwa-trowe." Five (cinco) sounds like "sink-oh." Six (seis) sounds like "saiss." Seven (siete) sounds like "see-ye-teh." Eight (ocho) sounds like it is written, or "oh-choh." Nine (nueve) sounds like "nwe-ve." Ten (diez) sounds like "dyess." Go through these numbers a few times without writing them in Spanish or English phonetics, as this may confuse the students.
Sing along with a song containing the numbers or watch a video, such as the "Pinball Number Song" in Spanish from Sesame Street. This will help younger students especially to remember the numbers.
Write the numeral one through 10 on the board, with the names in Spanish, just so the students get used to the phonetic pronunciation and the Spanish spelling. This will help them learn the correct pronunciation of other words in the future as well.
Have the students repeat after you the numbers 11 through 20 with the students. Again, do not write the Spanish or phonetics on the board until the students can confidently pronounce all of the numbers. The number 11 (once) sounds like "ohn-say;" 12 (doce) sounds like "doh-say;" 13 (trece) sounds like "tray-say;" 14 (catorce) sounds like "ka-tohr-say;" 15 (quince) sounds like "keen-say;" 16 (dieceseis) sounds like "dyess-ee-sayss;" 18 (dieciocho) sounds like "dyess-ee-oh-choh;" 19 (diecinueve) sounds like "dyess-ee-nwe-ve;" 20 (viente) sounds like "bayn-tay." Repeat these with the students until they can confidently say them before showing them the letter names written in Spanish.
Write the new numbers on the board in numerals, but keep pronouncing them phonetically, without the Spanish words.
Continue with the larger numbers, you do not need to pronounce every single number with English pronunciation, just 21 through 23 for students to get the idea of the sound of two digit numbers, the multiples of 10, 100, 200, and 300 so that they understand hundreds, 1,000 and 2,000 so that they understand thousands. The number 21 (veintiuno) is pronounced "bayn-teh-ooh-noh;" 22 (vientidos) is pronounced "bayn-teh-dose;" 23 (vientitres) is pronounced "bayn-tee-trace;" 30 (trienta) is pronounced "trayn-tah;" 40 (cuarenta) is pronounced "kwah-rayn-tah;" 50 (cincuenta) is pronounced "seen-kwayn-tah;" 60 (sesenta) is pronounced "say-sayn-tah;" 70 (setenta) is pronounced "say-tayn-tah;" 80 (ochenta) is pronounces "oh-chayn-tah;" 90 (noventa) is pronounced "noh-bayn-tah;" 100 (cien) is pronounced "seyehn;" 200 (doscientos) is pronounced "dos-syehn-tos;" 300 (trescientos) is pronounced "tres-syehn-tos;" 500 (quinientos) is pronounced "kee-nyehn-tos;" 1,000 (mil) is pronounced "meel;" 2,000 (dos mil) is pronounced "dose-meel."
Repeat all of the new numbers with the students. Have them pronounce all of the numbers along with a video that shows the numbers, written in both English and Spanish, to tie their learning of the numbers with English pronunciation in with the numbers as they are written in Spanish.