If you are using the clear 6mm and 4mm tubing. Its easier. The 4mm tube have a OD that fits nice and tight into the 6mm ID.

I fit a short piece of 4mm onto the fuel pump (about 15mm) and than slip the 6mm tube coming from the UAT over the 4mm line all the way onto the nipple on the fuel pump. Its a tight fit and holds very well.