I tried to learn about replenishment method but I found it too complicated to understand every step.
If you are using X-Tol:
1) Make up the 5 litres from the package;
2) Put 1 litre in a working solution bottle, labelled "working X-Tol". Put the rest in four separate 1 litre bottles labeled as "replenisher";
3) Develop your film in as much of the working solution as you need;
4) For each 4x5 sheet of film developed, transfer 20 ml of the replenisher into the working solution bottle. If you shoot roll film, use 70-80 ml per roll of 120 or 135-36;
5) When the developing is finished, pour the developer back into the working solution bottle. It will over-flow - discard that over-flow;
6) During any week that you don't develop film, pretend you developed 4 sheets of 4x5 - just discard 80 ml of working solution from the bottle, and top it up with replenisher;
7) When you get down to your last 1 litre bottle of replenisher, buy some more X-Tol, mix up the 5 litres, and divide it into 5 one litre bottles of replenisher. Keep using your old working solution, with the new replenisher.
If you start to see regular, slight under-development, increase slightly the amount you replenish per roll or sheet. If you start to see regular, slight over-development, decrease slightly the amount you replenish per roll or sheet.
T-Max RS works very similarly.