Please step by step cause I'm not good at it.

Open the for in design view, on the toolbox click the icon that looks like it has 2 little windows on the right side of it , then click and drag on the form to make a subform, resize it after your done if necessary, to enter into the subform double click on the white somewhere if it doesnt work the first time click anywhere on the main form and double click inside of it again and there you go, hope this helps.
Rpb

Another way is to create a form that you want to insert as a subform. Once created open the main form. Drag from the database window the form you want inserted as a subform, drop into your form, position and size as needed. Make sure to have a unique id in both forms to link the main and sub together.