You can do chicken or beef but since my kids prefer chicken I'll give you that with the variances for beef included as well.

Remove excess fat from chicken breasts or beef.
Slice the chicken breasts (or tri-tip cut of beef) into 1/2 wide strips roughly 1-2 inches long.
Pour a small amount of the marinade into the bottom of your crock pot. I do just enough to cover the bottom.
Start adding in pieces of chicken as you finish cutting up each breast.
Cut the bell peppers into slices lengthwise. Rinse off any seeds and cut off stem pieces. (If the strips are longer than about 2 inches I cut them in half.)
Add about a fourth of each pepper with each chicken breast.
Add more marinade on top of each group of breast and pepper strips; enough to cover the pile.
Repeat the process until all breasts and peppers are now in the crock pot.
Cover the contents with marinade until submerged or darn close to it. (Contents will settle as they cook.)

Set on low heat and cover. (Don't forget to plug in the crock pot. It's embarrassing and frustrating to forget this step.)
Stir every hour or so (I use a ladle with holes to allow the liquid to flow through. I try and bring the contents near the bottom to the top and vice versa to allow for more even cooking.)
Takes about 5-6 hours to cook on low setting. If you want to have them for lunch or you get a late start, set the crock pot on high for about 3 hours.

Remove from crock pot and serve with refried beans, and/or rice, cheese, salsa, sour cream, or whatever strikes your fancy. Oh, and tortillas.