-Why its important to have a good power supply
Your desktop/bench power supply is going to be a tool you use very often... From powering a board, to forcing a short circuit to generate heat. From personal experience, a very very cheap (diy) DCPS is a terrible idea. One of the first Macbook Airs I worked on had a shorted PCH. I though the 3A limit would be fine and it was variable up to 30V with auto current limiting - but guess what. That failed and the cheap-o power supply pumped out 30V onto my low voltage line. The cost of replacing that clients motherboard was more than a decent Power Supply off of amazon. I would honestly recomend a 10Amp 30V power supply. Imagine you are testing a 1.2v line at 1v looking for a short. If you are pumping all 10 amps through, that will only generate 10W of heat. It is a decent amount, but sometimes that is even hard to find if it is a large component shorted - like cpu.
-How to make MagSafe cable
Easy enough, you will need working cables - the power brick can be dead. If you see a client want to scrap an 85w MagSafe 1 or 2 charger because the cable is frayed at the brick - give them 5$ for it and use that wire. You want 85w cables. The data for the Whattage is in the MagSafe connector its self. The reason you want an 85w cable is because it will work for every macbook from the 11inch mba to the 2011 17inch mbp.
To make it, You need to strip the wires. There is a shielding that is surrounding an inner cable. The shielding layer is the negative line. The inner wire is the positive line. You just need to put some banana plugs on them and slap them into your dcps.