This procedure assumes you know how to install a memory SIMM correctly. In the case of the 25XT computer, you may also need to know how to correctly install DIP (Dual Inline Package) memory chips. Neither operation is too hard, but if you're not sure that you know how--stop and get some assistance or read up on the topic.
Take the time to reference the planar you have in your system...and what kind of system you have so that you know A) where the memory will go and how you can get to that location and B) what sort of memory you will need to get.
Check for your system type and planar here. This page has information on how to open your 25.
Your model 25 will take one of many types of memory based upon what exact model it is and what processor/planar combination you have installed.
This machine takes 2x30 pin SIMMs, each of 256K in size to get you a maximum of 640K RAM. It should happily accept any 30 pin 256K SIMM, but you have to put one in each slot. Most systems will be fully equipped with 640 K RAM. As with all PS/2s, you MUST use parity memory.
This machine can have a maximum of 4MB RAM installed on the planar board. Unfortunately it uses very unique SIMMs (similar to what is used on the 30-286 and some IBM SCSI controllers for PS/2) and a successful hack to a 9 chip, 30 pin SIMM would prove rather difficult without the proper soldering skills.
25SX has a single SIMM slot on the planar board, you may use up to an 8MB, 80NS 72 pin parity SIMM in this socket. Your planar will have 1 or 4MB of RAM soldered to the board permanently, giving you 9 or 12MB of RAM total respectively.
There is an outline for a SIMM socket on your planar, but you can forget about putting a socket there. The one person I have had try this had the machine fail to work right with a SIMM in the newly installed socket.
7386 and EduQuest 30/40 systems
The 7386 and EduQuests take industry-standard 30-pin parity SIMMs with capacities up to 4MB per SIMM. Your EduQuest planar (not the 7386) will have some RAM soldered down already much as the 25SX does. It will be 1 or 4MB and there is a table printed on your planar where hopefully one of the two possible soldered memory configurations has been checked off.