Line by line read between different worksheets

I am creating VBA code for an Excel worksheet that asks the user for heights of key components then is required to fill the gaps between the components with spacers as read from a second sheet. It has to be a second sheet because this sheet is completed else where by a different user at a different time. What I want is to be able to read a line in from the source sheet, check that adding it to the previous piece still fits between the components and if it does paste it in and read the next line of the source document and repeat. Making the fit decision I can do, however its the line by line reading between sheets I am unsure of. Can anyone help.